The -Wredundant-move warning is C++ only

Summary:
Prevent the compiler message:

warning: command-line option ‘-Wredundant-move’ is valid for C++/ObjC++
but not for C

Test Plan:

ninja check

Check the message is gone.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6284