include: use '#pragma once' everywhere

Change-Id: Iecc1754caae401aca28444d3e3d0cca04b398f26
diff --git a/include/osmocom/core/sercomm.h b/include/osmocom/core/sercomm.h
index 072f4d9..38e6271 100644
--- a/include/osmocom/core/sercomm.h
+++ b/include/osmocom/core/sercomm.h
@@ -2,8 +2,7 @@
  *  Osmocom Sercomm HDLC (de)multiplex.
  */
 
-#ifndef _SERCOMM_H
-#define _SERCOMM_H
+#pragma once
 
 #include <osmocom/core/msgb.h>
 
@@ -110,5 +109,3 @@
 }
 
 /*! @} */
-
-#endif /* _SERCOMM_H */