message: Add C linkage to crypto functions
diff: #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
  