plymouth: building documatation sometimes fails to download so disable it

| 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


| Makefile:622: recipe for target 'plymouth.1' failed

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