fix gcc 4.7.0 warning related to enum type

This amends commit 7bd9efc7.

We changed away from using unsigned here because it caused a warning
on Windows.  Using the more specific type, however, is causing a
warning on gcc-4.7 (it notices that the switch statement contains
values not present in the specified enum type).  So we're switching
back to unsigned here while keeping the type casts and hoping that
keeps Windows happy.

src/switch_rtp.c

@@ -692,7 +692,7 @@ 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, zrtp_security_event_t event)
+static void zrtp_event_callback(zrtp_stream_t *stream, unsigned event)
 {
 	switch_rtp_t *rtp_session = zrtp_stream_get_userdata(stream);
 	zrtp_session_info_t zrtp_session_info;