gamin: fix compilation with glib 2.30

G_CONST_RETURN is deprecated in glib 2.30 so remove occurences
to avoid build failures.

Patch taken from http://patches.openembedded.org/patch/13079/



Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>