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.

server/modules/monitor/mysqlmon.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _MYSQLMON_H
 #define _MYSQLMON_H
 /*
@@ -13,6 +14,7 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -30,6 +32,8 @@
 #include <maxscale/externcmd.h>
 #include <maxscale/hashtable.h>
 
+MXS_BEGIN_DECLS
+
 /**
  * @file mysqlmon.h - The MySQL monitor
  *
@@ -80,4 +84,6 @@ typedef struct
     bool warn_failover; /**< Log a warning when failover happens */
 } MYSQL_MONITOR;
 
+MXS_END_DECLS
+
 #endif