msc/gsm_04_80.h: use '#pragma once' instead of includes

This is a common include guard way for Osmocom projects.

Change-Id: I5a12a742719d4deba0b9dfa8bb330c1986a727f6

include/osmocom/msc/gsm_04_80.h

@@ -1,5 +1,4 @@
-#ifndef _GSM_04_80_H
-#define _GSM_04_80_H
+#pragma once
 
 #include <osmocom/core/msgb.h>
 #include <osmocom/gsm/protocol/gsm_04_80.h>
@@ -17,5 +16,3 @@ int gsm0480_send_ussd_reject(struct gsm_subscriber_connection *conn,
 int msc_send_ussd_notify(struct gsm_subscriber_connection *conn, int level,
 			 const char *text);
 int msc_send_ussd_release_complete(struct gsm_subscriber_connection *conn);
-
-#endif