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/gw_ssl.h

@@ -1,5 +1,6 @@
-#ifndef _GW_SSL_H
-#define _GW_SSL_H
+#pragma once
+#ifndef _MAXSCALE_GW_SSL_H
+#define _MAXSCALE_GW_SSL_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -27,12 +28,15 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/gw_protocol.h>
 #include <openssl/crypto.h>
 #include <openssl/ssl.h>
 #include <openssl/err.h>
 #include <openssl/dh.h>
 
+MXS_BEGIN_DECLS
+
 struct dcb;
 
 typedef enum ssl_method_type
@@ -78,4 +82,6 @@ bool ssl_required_by_dcb(struct dcb *dcb);
 bool ssl_required_but_not_negotiated(struct dcb *dcb);
 const char* ssl_method_type_to_string(ssl_method_type_t method_type);
 
+MXS_END_DECLS
+
 #endif /* _GW_SSL_H */