fix function-pointer storage

the C standard says that you're not allowed to cast a function pointer to
something else

 6.3.2.3 §8:

A pointer to a function of one type may be converted to a pointer
to a function of another type and back again

src/common.h

@@ -107,7 +107,7 @@ typedef struct _xmpp_handlist_t xmpp_handlist_t;
 struct _xmpp_handlist_t {
     /* common members */
     int user_handler;
-    void *handler;
+    int (*handler)();
     void *userdata;
     int enabled; /* handlers are added disabled and enabled after the
                   * handler chain is processed to prevent stanzas from