libx11: skip self if x11 not in DISTRO_FEATURES

This stops (most) x11 packages leaking into a non-x11 distro by mistake.

Signed-off-by: Phil Blundell <philb@gnu.org>

The patch was imported from the oe-core git server
(git://git.openembedded.org/openembedded-core

) as of commit id
2f94ff4fdb66755ba2448794baea19315ff96273.

Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>