zenity: fix build

error message was:
| warning: failed to load external entity "/usr/share/xml/gnome/xslt/docbook/omf/db2omf.xsl"
| cannot parse /usr/share/xml/gnome/xslt/docbook/omf/db2omf.xsl
| make[2]: *** [zenity-C.omf] Error 1

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