osm_gps_map_image_get_zorder: Change return type to "gint"

... instead of "const gint".

The returned value is a scalar, not a pointer, therefore it's copied.
The "const" qualifier is unneeded and even breaks the build with older
GCC such as 4.2.
Merge pull request #20 from dumbbell/fix-with-gcc42

osm_gps_map_image_get_zorder: Change return type to "gint"