Add C linkage to crypto functions

contracts/eoslib/crypto.h

 #pragma once
 #include <eoslib/types.h>
-
+extern "C" {
 /**
  *  This method is implemented as:
  *
@@ -16,4 +16,4 @@ void assert_sha256( char* data, uint32_t length, const uint256* hash );
  *  Calculates sha256( data,length) and stores result in memory pointed to by hash 
  */
 void sha256( char* data, uint32_t length, uint256* hash );
-
+}
\ No newline at end of file