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/secrets.h

@@ -1,5 +1,6 @@
-#ifndef _SECRETS_H
-#define _SECRETS_H
+#pragma once
+#ifndef _MAXSCALE_SECRETS_H
+#define _MAXSCALE_SECRETS_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -25,6 +26,7 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
@@ -32,9 +34,10 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <errno.h>
-
 #include <openssl/aes.h>
 
+MXS_BEGIN_DECLS
+
 #define MAXSCALE_KEYLEN 32
 #define MAXSCALE_IV_LEN 16
 
@@ -56,4 +59,6 @@ extern int  secrets_writeKeys(const char *directory);
 extern char *decryptPassword(const char *);
 extern char *encryptPassword(const char*, const char *);
 
+MXS_END_DECLS
+
 #endif