add some casts to make things work with both gcc and VS

This amends commit 7bd9efc7.

Using the more explicit typedef causes gcc to warn about the
incompatible types for the reasons noted in the earlier commit
message.  This is why we just used the unsigned type earlier so we
could avoid all this casting.

The things we do to make Windows happy...

src/switch_rtp.c

@@ -864,9 +864,9 @@ SWITCH_DECLARE(void) switch_rtp_init(switch_memory_pool_t *pool)
 		zrtp_zstrcpyc((zrtp_stringn_t*)zrtp_config.def_cache_path.buffer, zrtp_cache_path);
 		zrtp_config.def_cache_path.length = (uint16_t)strlen(zrtp_cache_path);
 		zrtp_config.def_cache_path.max_length = 255;
-		zrtp_config.cb.event_cb.on_zrtp_protocol_event = zrtp_event_callback;
+		zrtp_config.cb.event_cb.on_zrtp_protocol_event = (void (*)(zrtp_stream_t*,zrtp_protocol_event_t))zrtp_event_callback;
 		zrtp_config.cb.misc_cb.on_send_packet = zrtp_send_rtp_callback;
-		zrtp_config.cb.event_cb.on_zrtp_security_event = zrtp_event_callback;
+		zrtp_config.cb.event_cb.on_zrtp_security_event = (void (*)(zrtp_stream_t*,zrtp_security_event_t))zrtp_event_callback;
 
 		zrtp_log_set_log_engine((zrtp_log_engine *) zrtp_logger);
 		zrtp_log_set_level(4);