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