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/filter.h

@@ -1,5 +1,6 @@
-#ifndef _FILTER_H
-#define _FILTER_H
+#pragma once
+#ifndef _MAXSCALE_FILTER_H
+#define _MAXSCALE_FILTER_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -22,11 +23,15 @@
  * 27/05/2014   Mark Riddoch            Initial implementation
  *
  */
+
+#include <maxscale/cdefs.h>
 #include <maxscale/dcb.h>
 #include <maxscale/session.h>
 #include <maxscale/buffer.h>
 #include <stdint.h>
 
+MXS_BEGIN_DECLS
+
 /**
  * The FILTER handle points to module specific data, so the best we can do
  * is to make it a void * externally.
@@ -116,4 +121,6 @@ void dprintAllFilters(DCB *);
 void dprintFilter(DCB *, FILTER_DEF *);
 void dListFilters(DCB *);
 
+MXS_END_DECLS
+
 #endif