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/authenticator/gssapi_auth.h

@@ -1,3 +1,6 @@
+#pragma once
+#ifndef _GSSAPI_AUTH_H
+#define _GSSAPI_AUTH_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -11,13 +14,13 @@
  * Public License.
  */
 
-#ifndef _GSSAPI_AUTH_H
-#define _GSSAPI_AUTH_H
-
+#include <maxscale/cdefs.h>
 #include <stdint.h>
 #include <stddef.h>
 #include <gssapi.h>
 
+MXS_BEGIN_DECLS
+
 /** Client auth plugin name */
 static const char auth_plugin_name[] = "auth_gssapi_client";
 
@@ -48,4 +51,6 @@ void gssapi_auth_free(void *data);
 /** Report GSSAPI errors */
 void report_error(OM_uint32 major, OM_uint32 minor);
 
+MXS_END_DECLS
+
 #endif