cogl: use G_BEGIN_DECLS instead of COGL_BEGIN_DECLS

This macro was introduced so as to be able to be built without GLib.
However, this feature was long ago removed, and in Mutter we depend on
it anyway, so let's get rid of it in favor of more consistency.

cogl/cogl-path/cogl-path-functions.h

@@ -43,7 +43,7 @@
 #endif
 #include <glib-object.h>
 
-COGL_BEGIN_DECLS
+G_BEGIN_DECLS
 
 /**
  * cogl_path_get_gtype:
@@ -535,7 +535,7 @@ COGL_DEPRECATED_FOR (cogl_framebuffer_push_path_clip)
 void
 cogl_clip_push_from_path (CoglPath *path);
 
-COGL_END_DECLS
+G_END_DECLS
 
 #endif /* __COGL_PATH_FUNCTIONS_H__ */