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/routing/maxinfo/maxinfo.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _MAXINFO_H
 #define _MAXINFO_H
 /*
@@ -12,10 +13,13 @@
  * of this software will be governed by version 2 or later of the General
  * Public License.
  */
+#include <maxscale/cdefs.h>
 #include <maxscale/service.h>
 #include <maxscale/session.h>
 #include <maxscale/spinlock.h>
 
+MXS_BEGIN_DECLS
+
 /**
  * @file maxinfo.h The MaxScale information schema provider
  *
@@ -138,4 +142,7 @@ extern void         maxinfo_send_parse_error(DCB *, char *, PARSE_ERROR);
 extern void     maxinfo_send_error(DCB *, int, char  *);
 extern RESULTSET    *maxinfo_variables();
 extern RESULTSET    *maxinfo_status();
+
+MXS_END_DECLS
+
 #endif