Fix a Clang -Wshadow warning in rcu.h

Summary: As per title

Test Plan:
Using clang as a compiler:

make check

Ensure there is no -Wshadow warning

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2491