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/memlog.h

@@ -1,5 +1,6 @@
-#ifndef _MEMLOG_H
-#define _MEMLOG_H
+#pragma once
+#ifndef _MAXSCALE_MEMLOG_H
+#define _MAXSCALE_MEMLOG_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -24,8 +25,12 @@
  *
  * @endverbatim
  */
+
+#include <maxscale/cdefs.h>
 #include <maxscale/spinlock.h>
 
+MXS_BEGIN_DECLS
+
 typedef enum { ML_INT, ML_LONG, ML_LONGLONG, ML_STRING } MEMLOGTYPE;
 
 typedef struct memlog
@@ -59,4 +64,6 @@ extern void    memlog_log(MEMLOG *, void *);
 extern void    memlog_flush_all();
 extern void    memlog_flush(MEMLOG *);
 
+MXS_END_DECLS
+
 #endif