Revert "gconf.bbclass: remove --disable-schemas-install"

Revert "gconf.bbclass: remove --disable-schemas-install"

This reverts commit 84fa245b1d1e7935094e3b8a452d69461dc3dcbb.

Ross Burton comments that:

*Some* packages have migrated away from gconf to gsettings.  Not all
packages have done this, so anything still using gconf is now broken.
Note that almost nothing in oe-core uses gsettings as we're still
using the GTK+ 2 stack generally.

The problem is that people are using gnome.bbclass which is a kitchen
sink class, inheriting many other classes that may or may not be used.
To resolve this warning gnome.bbclass should extend the sanity check
whitelist with --disable-schemas-install as it is the class causing
gconf.bbclass to be inherited on recipes that don't use gconf.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>