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/hk_heartbeat.h

@@ -1,5 +1,6 @@
-#ifndef _HK_HEARTBEAT_H
-#define _HK_HEARTBEAT_H
+#pragma once
+#ifndef _MAXSCALE_HK_HEARTBEAT_H
+#define _MAXSCALE_HK_HEARTBEAT_H
 
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
@@ -14,6 +15,10 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
+
+MXS_BEGIN_DECLS
+
 /**
  * The global housekeeper heartbeat value. This value is incremented
  * every 100 milliseconds and may be used for crude timing etc.
@@ -21,4 +26,6 @@
 
 extern long hkheartbeat;
 
+MXS_END_DECLS
+
 #endif