libgtk2: add support for some optional X dependencies

libgtk2 can optionally use a number of features from additional X
extensions, so we add support for those, to ensure that Gtk gets built
after those extensions if they have been enabled in the Buildroot
configuration.

The extensions are Xinerama, Xinput, Xrandr, Xcursor, Xfixes,
Xcomposite and Xdamage.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>