Add g_type_ensure() and use it rather than playing games with volatile

https://bugzilla.gnome.org/show_bug.cgi?id=605976

gio/gsocketconnection.c

@@ -602,12 +602,10 @@ g_socket_connection_factory_register_type (GType         g_type,
 static void
 init_builtin_types (void)
 {
-  volatile GType a_type;
 #ifndef G_OS_WIN32
-  a_type = g_unix_connection_get_type ();
+  g_type_ensure (G_TYPE_UNIX_CONNECTION);
 #endif
-  a_type = g_tcp_connection_get_type ();
-  (a_type); /* To avoid -Wunused-but-set-variable */
+  g_type_ensure (G_TYPE_TCP_CONNECTION);
 }
 
 /**