Fix the OSX build

Summary:
Removing 2 fields from the RPCResult created one of these cases where we
need double brackets to differentiate from structure construction.

There is no change in behavior.

Test Plan:
Cross build for OSX.

./bitcoin-cli help getmempoolentry

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13148