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/include/mysql_auth.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _MYSQL_AUTH_H
 #define _MYSQL_AUTH_H
 /*
@@ -23,11 +24,14 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/dcb.h>
 #include <maxscale/buffer.h>
 #include <stdint.h>
 #include <maxscale/protocol/mysql.h>
 
+MXS_BEGIN_DECLS
+
 int gw_check_mysql_scramble_data(DCB *dcb,
                                  uint8_t *token,
                                  unsigned int token_len,
@@ -41,4 +45,6 @@ int gw_find_mysql_user_password_sha1(
     uint8_t *gateway_password,
     DCB *dcb);
 
+MXS_END_DECLS
+
 #endif /** _MYSQL_AUTH_H */