xorg: needs thread support

libpthread-stubs, libX11, and a few more xorg packages use threads. Since
almost all xorg packages depend on libX11 directory or indirectly, and
since the remaining ones are pretty useless on their own, just require
threads for xorg as a whole.

The thread dependency is kept in libpthread-stubs, because that package
will move out of the x11r7 directory later (pending patch by Spenser
Gilliland), so the dependency on threads will be required then.

Fixes:
  http://autobuild.buildroot.org/results/609/6099baac4bb469ae18aab6512233db25183eaabd/



[Arnout: disable all of xorg, correct comment]
Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>