build: add missing includes to header files

This change adds necessary `#include` directives that were previously
omitted in some header files. Without these includes, compilation
could fail or produce undefined behavior when the headers are used
in isolation (i.e., before their dependencies are included elsewhere).

Ensures better modularity and reliability of header usage.

src/xmpp/ox.h

@@ -7,6 +7,8 @@
  * SPDX-License-Identifier: GPL-3.0-or-later WITH OpenSSL-exception
  */
 
+#include "glib.h"
+
 /*!
  * \page OX OX Implementation
  *