package: don't select libgtk2 as it doesn't know what backend to use

package: don't select libgtk2 as it doesn't know what backend to use

libgtk2 can either use the X11 or directfb backend, depending on what
is selected - But if other packages 'select' libgtk2 there is no
guarantee that any backends are enabled, breaking the build.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>