style: remove extra const keyword from interfaces

Const variables in prototypes don't add much value, but make the code
larger and redundant. Remove these const keywords.
Note, this doesn't apply to pointers to const memory.

src/event.c

@@ -73,7 +73,7 @@
  *
  *  @ingroup EventLoop
  */
-void xmpp_run_once(xmpp_ctx_t *ctx, const unsigned long timeout)
+void xmpp_run_once(xmpp_ctx_t *ctx, unsigned long timeout)
 {
     xmpp_connlist_t *connitem;
     xmpp_conn_t *conn;