C implementation: Fix compiler warnings

The C implementation currently has compiler warnings.

https://github.com/google/open-location-code/actions/runs/12468691641/job/34800362797

These are related to non-standard extensions [implemented in our C implementation](https://github.com/google/open-location-code/blob/main/c/src/olc.h#L57-L58) instead of what [our specification requires](https://github.com/google/open-location-code/blob/main/Documentation/Specification/specification.md?plain=1#L185).