gvfs: disable build of documentation to fix build in some envirionments

man/Makfile.am make is trying to download

http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl

which sometimes fails with:

make[2]: Entering directory `/home/peter/oe-core/tmp-glibc/work/cortexa9t2hf-vfp-neon-angstrom-linux-gnueabi/gvfs/1.24.2-r0/build/man'
/home/peter/oe-core/tmp-glibc/sysroots/x86_64-linux/usr/bin/xsltproc --nonet --stringparam man.output.quietly 1 --stringparam funcsynopsis.style ansi --stringparam man.th.extra1.suppress 1 --stringparam man.authors.section.enabled 0 --stringparam man.copyright.section.enabled 0 http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl /home/peter/oe-core/tmp-glibc/work/cortexa9t2hf-vfp-neon-angstrom-linux-gnueabi/gvfs/1.24.2-r0/gvfs-1.24.2/man/gvfs-cat.xml
/home/peter/oe-core/tmp-glibc/sysroots/x86_64-linux/usr/bin/xsltproc --nonet --stringparam man.output.quietly 1 --stringparam funcsynopsis.style ansi --stringparam man.th.extra1.suppress 1 --stringparam man.authors.section.enabled 0 --stringparam man.copyright.section.enabled 0 http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl /home/peter/oe-core/tmp-glibc/work/cortexa9t2hf-vfp-neon-angstrom-linux-gnueabi/gvfs/1.24.2-r0/gvfs-1.24.2/man/gvfs-copy.xml
/home/peter/oe-core/tmp-glibc/sysroots/x86_64-linux/usr/bin/xsltproc --nonet --stringparam man.output.quietly 1 --stringparam funcsynopsis.style ansi --stringparam man.th.extra1.suppress 1 --stringparam man.authors.section.enabled 0 --stringparam man.copyright.section.enabled 0 http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl /home/peter/oe-core/tmp-glibc/work/cortexa9t2hf-vfp-neon-angstrom-linux-gnueabi/gvfs/1.24.2-r0/gvfs-1.24.2/man/gvfs-info.xml
/home/peter/oe-core/tmp-glibc/sysroots/x86_64-linux/usr/bin/xsltproc --nonet --stringparam man.output.quietly 1 --stringparam funcsynopsis.style ansi --stringparam man.th.extra1.suppress 1 --stringparam man.authors.section.enabled 0 --stringparam man.copyright.section.enabled 0 http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl /home/peter/oe-core/tmp-glibc/work/cortexa9t2hf-vfp-neon-angstrom-linux-gnueabi/gvfs/1.24.2-r0/gvfs-1.24.2/man/gvfs-ls.xml
I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
make[2]: *** [gvfs-cat.1] Error 4
make[2]: *** Waiting for unfinished jobs....
make[2]: *** [gvfs-copy.1] Error 4
I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
make[2]: *** [gvfs-info.1] Error 4
I/O error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl


make[2]: *** [gvfs-ls.1] Error 4
make[2]: Leaving directory `/home/peter/oe-core/tmp-glibc/work/cortexa9t2hf-vfp-neon-angstrom-linux-gnueabi/gvfs/1.24.2-r0/build/man'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/peter/oe-core/tmp-glibc/work/cortexa9t2hf-vfp-neon-angstrom-linux-gnueabi/gvfs/1.24.2-r0/build'
make: *** [all] Error 2
ERROR: oe_runmake failed

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>