include util.h to get SecureString definition.

src/crypter.h

@@ -4,6 +4,7 @@
 #ifndef __CRYPTER_H__
 #define __CRYPTER_H__
 
+#include "util.h" /* for SecureString */
 #include "key.h"
 
 const unsigned int WALLET_CRYPTO_KEY_SIZE = 32;