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/housekeeper.h

@@ -1,5 +1,6 @@
-#ifndef _HOUSEKEEPER_H
-#define _HOUSEKEEPER_H
+#pragma once
+#ifndef _MAXSCALE_HOUSEKEEPER_H
+#define _MAXSCALE_HOUSEKEEPER_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -12,9 +13,14 @@
  * of this software will be governed by version 2 or later of the General
  * Public License.
  */
+
+#include <maxscale/cdefs.h>
 #include <time.h>
 #include <maxscale/dcb.h>
 #include <maxscale/hk_heartbeat.h>
+
+MXS_BEGIN_DECLS
+
 /**
  * @file housekeeper.h A mechanism to have task run periodically
  *
@@ -54,4 +60,6 @@ extern int  hktask_remove(const char *name);
 extern void hkshutdown();
 extern void hkshow_tasks(DCB *pdcb);
 
+MXS_END_DECLS
+
 #endif