giomodule: return a copy of module name

This is a regression from commit 6dedc0.

The clients expect to free the received module name, so the function
must return a copy.

https://bugzilla.gnome.org/show_bug.cgi?id=756952

gio/giomodule.c

@@ -1036,7 +1036,7 @@ get_gio_module_dir (void)
 #endif
       g_free (install_dir);
 #else
-      module_dir = GIO_MODULE_DIR;
+      module_dir = g_strdup (GIO_MODULE_DIR);
 #endif
     }