bugfix: decode_mnemonic was writing to wrong stream

In decode_mnemonic() the output is written to the output stream 'ss'
with the exception of one std::endl which was written to std::cout.
This also broke compilation on some platforms which require iostream
to be explicitly included to access std::cout.
Merge pull request #15 from rdks/rdks_fix_mnemonic_write_wrong_stream

bugfix: decode_mnemonic was writing to wrong stream