Add share/rpcuser to dist. source code archive

Summary:

As the legacy rpcuser and rpcpassword are deprected since 0.12.0, we
should actually include the script to generate the new auth pair in the
distributed source code archive.

Backport of core PR11530
https://github.com/bitcoin/bitcoin/pull/11530/files

Test Plan:

make dist

Check the script is part of the generated archive

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3554