Silence gcc overloaded-virtual warning in FieldBackedMapImpl

[krinkinmu]

gcc complains about FieldBackedMapImpl::ListKeys when overloaded-virtual check is enabled. When using -Werror, warnings like this are converted to errors breaking builds.

That's what happened to gcc build of Envoy a while back. The issue in Envoy was fixed by introducing a patch for cel-cpp that was applied to the source during Envoy build (see
envoyproxy/envoy#31814).

Unfortunately during cel-cpp version update in Envoy instead of updating the patch, dropping the parts that are not needed anymore, we dropped the whole cep-cpp patch all together in
envoyproxy/envoy#36661. And now gcc can't build Envoy again.

This change makes the change to silence the gcc warning in the cel-cpp codebase, which hopefully will be a bit more durable than maintaing a patch on the Envoy side.

The change is not very invasive (just a using declaration), plus I can see at least another place in the code that does the same to deal with gcc warning (see

cel-cpp/tools/flatbuffers_backed_impl.h

Lines 27 to 28 in 61d0334

	// Import base class signatures to bypass GCC warning/error.
	using CelMap::ListKeys;

). With that in mind, would you be open to include this change in the cel-cpp upstream repository?

Thank you.

[krinkinmu]

I see that it was fixed, thank you!