gstmodule: fix warning when building against python2

PyMapping_GetItemString’ discards ‘const’ qualifier from pointer target type

https://bugzilla.gnome.org/show_bug.cgi?id=796093

gi/overrides/gstmodule.c

@@ -66,7 +66,7 @@ GST_DEBUG_CATEGORY_STATIC (pygst_debug);
 #define GST_CAT_DEFAULT pygst_debug
 
 static PyObject *
-gi_gst_get_type (const gchar * type_name)
+gi_gst_get_type (gchar * type_name)
 {
   PyObject *module, *dict;