Use void* to store function pointer when compiled as C23.

[0] decided that `rettype (*foo)();` must now be interpreted as
`rettype (*foo)(void);`. Luckily it also allows now to store function
pointers in a `void*` (c.f. Ch. J.5.7).

[0]: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

src/handler.c

@@ -25,8 +25,6 @@
 #include "common.h"
 #include "ostypes.h"
 
-typedef int (*xmpp_void_handler)();
-
 /* Remove item from the list pointed by head, but don't free it.
  * There can be a situation when user's handler deletes another handler which
  * is the previous in the list. handler_fire_stanza() and handler_fire_timed()