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/platform.h

@@ -1,5 +1,6 @@
-#ifndef _PLATFORM_H
-#define _PLATFORM_H
+#pragma once
+#ifndef _MAXSCALE_PLATFORM_H
+#define _MAXSCALE_PLATFORM_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -13,6 +14,10 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
+
+MXS_BEGIN_DECLS
+
 #if !defined(__cplusplus)
 
 #if __STDC_VERSION__ >= 201112
@@ -48,4 +53,6 @@
 
 #endif // __cplusplus
 
+MXS_END_DECLS
+
 #endif // _PLATFORM_H