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/statistics.h

@@ -1,5 +1,6 @@
-#ifndef _STATISTICS_HG
-#define _STATISTICS_HG
+#pragma once
+#ifndef _MAXSCALE_STATISTICS_HG
+#define _MAXSCALE_STATISTICS_HG
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -25,8 +26,11 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <stdint.h>
 
+MXS_BEGIN_DECLS
+
 typedef void* ts_stats_t;
 
 /** stats_init should be called only once */
@@ -68,4 +72,6 @@ ts_stats_set(ts_stats_t stats, int value, int thread_id)
     ((int64_t*)stats)[thread_id] = value;
 }
 
+MXS_END_DECLS
+
 #endif