use the typedef for an enum rather than unsigned

The callback here receives both zrtp_protocol_event_t and
zrtp_security_event_t events.  The latter is numbered contiguously to
the former.  Still, it's a bit confusing to have to pick one as a
type, but apparently this fixes a warning on Visual Studio on Windows.

Thanks to Jeff Lenk.

src/switch_rtp.c

@@ -688,7 +688,7 @@ static int zrtp_send_rtp_callback(const zrtp_stream_t *stream, char *rtp_packet,
 	return status;
 }
 
-static void zrtp_event_callback(zrtp_stream_t *stream, unsigned event)
+static void zrtp_event_callback(zrtp_stream_t *stream, zrtp_security_event_t event)
 {
 	switch_rtp_t *rtp_session = zrtp_stream_get_userdata(stream);
 	zrtp_session_info_t zrtp_session_info;