build: Prototype GType accessors for private classes

Otherwise we fail to build with -Werror=missing-prototypes.

https://bugzilla.gnome.org/show_bug.cgi?id=687385

tests/refcount/properties3.c

@@ -31,6 +31,7 @@ struct _GTestClass
   GObjectClass parent_class;
 };
 
+static GType my_test_get_type (void);
 G_DEFINE_TYPE (GTest, my_test, G_TYPE_OBJECT);
 
 static volatile gboolean stopping;