Merge #12787: rpc: Adjust ifdef to avoid unreachable code

Summary:
61f8298 rpc: Adjust ifdef to avoid unreachable code (practicalswift)

Pull request description:

Adjust `ifdef` to avoid unreachable code.

Introduced in 1e79c055cd30d21ba5f8c7f81ef911d5d4e295a8.

Tree-SHA512: c775cc9181e4034f26c5b219974e06886435275933249b169d2bc8bc98f639c4027e1e7d991f43bded62146a141acee6d3be1f2b313042d9bbc0a5d2e71d6c7c

Backport of Core PR12787
https://github.com/bitcoin/bitcoin/pull/12787/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3902