gtype: mark the inline functions in G_DECLARE_*_TYPE() as UNUSED
gtype: mark the inline functions in G_DECLARE_*_TYPE() as UNUSED

clang will warn about static inline functions where gcc will not.  Fixes
-Werror=unused-function with clang in dependant projects.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib