A missing Init::makeMining implementation was causing internal code using the
mining interface (like the `waitforblockheight` RPC method) to not work when
running inside the `bitcoin-gui` binary. It was working the other bitcoin
binaries ('bitocind`, `bitcoin-qt`, and `bitcoin-node`) because they
implmemented `Init::makeMining` methods in commit
8ecb681678 from #30200, but the `bitcoin-gui`
init class was forgotten in that change.
This bug was reported by Matthew Zipkin <pinheadmz@gmail.com>
https://github.com/bitcoin/bitcoin/pull/32297#pullrequestreview-2932651216
4565cff72c