Cleanup header files

- All now include maxscale/cdefs.h as the very first file.
- MXS_[BEGIN|END]_DECLS added to all C-headers.
  Strictly speaking not necessary for private headers, but
  does not hurt either.
- Include guards moved to the very top of the file.
- #pragma once added.

include/maxscale/gw_protocol.h

@@ -1,5 +1,6 @@
-#ifndef GW_PROTOCOL_H
-#define GW_PROTOCOL_H
+#pragma once
+#ifndef _MAXSCALE_GW_PROTOCOL_H
+#define _MAXSCALE_GW_PROTOCOL_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -28,8 +29,11 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/buffer.h>
 
+MXS_BEGIN_DECLS
+
 struct dcb;
 struct server;
 struct session;
@@ -81,6 +85,7 @@ typedef struct gw_protocol
  */
 #define GWPROTOCOL_VERSION      {1, 1, 0}
 
+MXS_END_DECLS
 
 #endif /* GW_PROTOCOL_H */