Add connection_get_profanity_identifier stub

And move defintion to xmpp.h

tests/unittests/xmpp/stub_connection.c

@@ -0,0 +1,7 @@
+#include "xmpp/message.h"
+
+ProfMessage *message_init(void) {
+    return NULL;
+}
+
+void message_free(ProfMessage *message) {}