The -Wredundant-move warning is C++ only
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

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