bugfix: decode_mnemonic was writing to wrong stream

While I was investigation build issues on Arch Linux I came across
this issue in decode_mnemonic().
The output of the method is written to the stream 'ss', but one endl
is written to std::cout.
To me this looks like a copy-paste error from an earlier implementation.
In addition it breaks compilation on some platforms, as it it were
actually supposed to write to cout an explicit #include <iostream> would
be needed.