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/queuemanager.h

@@ -1,5 +1,6 @@
-#ifndef _QUEUEMANAGER_H
-#define _QUEUEMANAGER_H
+#pragma once
+#ifndef _MAXSCALE_QUEUEMANAGER_H
+#define _MAXSCALE_QUEUEMANAGER_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -26,9 +27,12 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <stdbool.h>
 #include <maxscale/spinlock.h>
 
+MXS_BEGIN_DECLS
+
 #define CONNECTION_QUEUE_LIMIT 1000
 
 typedef struct queue_entry
@@ -67,4 +71,6 @@ mxs_queue_count(QUEUE_CONFIG *queue_config)
     return count < 0 ? (count + queue_config->queue_limit + 1): count;
 }
 
+MXS_END_DECLS
+
 #endif /* QUEUEMANAGER_H */