[gpm] Dbus return values in 2.20.1

[Seán de Búrca <leftmostcat gmail com>]

The ChangeLog for 2.20.1 states that the Dbus return values have been
fixed. I upgraded and discovered that the problem persists. A bit of
quick investigation shows that the changes were made to trunk after the
gnome-2-20 branch was created and were never backported. It should be a
trivial matter to fix and I can make the changes in subversion if you'd
like me to do so. Thanks.

Seán de Búrca