Fix protobuf linking when building without the wallet

Summary:
This last minute modification of D4619: https://reviews.bitcoinabc.org/D4619?vs=14737&id=15017#toc broke the build without the wallet (regardless of the value for ENABLE_BIP70). Digging into my build history, I found I accidentally tested that change with -DBUILD_WITHWALLET=OFF instead of the correct -DBUILD_BITCOIN_WALLET=OFF

This patch fixes the linking such that the build works as expected when the wallet is disabled but BIP70 is still enabled.

Test Plan:

cmake -GNinja -DBUILD_BITCOIN_WALLET=OFF ..
ninja check

Also run build-without-wallet on CI for sanity.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4797