gpsd: dbus interface requires dbus-glib

gpsd uses dbus-glib as the dbus interface, so it should only be built if
libglib2 has been selected.  To simplify things, build dbus support only
if dbus-glib is selected.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>