package/x11r7/xserver_xorg-server: select xlib_libxshmfence required by...

package/x11r7/xserver_xorg-server: select xlib_libxshmfence required by dri3proto support dependencies

When BR2_PACKAGE_XPROTO_DRI3PROTO and
BR2_PACKAGE_XSERVER_XORG_SERVER_MODULAR are selected, xlib_libxshmfence
package build is triggered at Makefile level.

Since 28f67899, the build
stop if a package build is trigged at Makefile level without
being selected at Config.in level.

Fixes:
http://autobuild.buildroot.net/results/a25/a253d19191dbc8bcc54f9e9d7fddae03d6695b4b/



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>