ussd.h: cosmetic: use '#pragma once' include guard

Change-Id: I0e78aa946b8e6bc7c31534751b2ee5c867bfd490
diff --git a/include/osmocom/msc/ussd.h b/include/osmocom/msc/ussd.h
index 84a3d6a..bc4227b 100644
--- a/include/osmocom/msc/ussd.h
+++ b/include/osmocom/msc/ussd.h
@@ -1,8 +1,5 @@
-#ifndef _USSD_H
-#define _USSD_H
+#pragma once
 
 #include <osmocom/core/msgb.h>
 
 int handle_rcv_ussd(struct gsm_subscriber_connection *conn, struct msgb *msg);
-
-#endif