libgtk2: mark DirectFB backend as deprecated

libgtk2 on DirectFB is deprecated because it is no longer supported in
recent versions of Gtk. We will remove support for Gtk over DirectFB
in the next Buildroot version unless support for DirectFB in mainline
Gtk is improved in the mean time.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>