Use .pb.h instead of .proto.h from Rust cc thunks.

The .proto.h is not emitted by default from the cc generator in OSS, rather than try to force it to emit we can just use the .pb.h

PiperOrigin-RevId: 676464651

src/google/protobuf/compiler/rust/naming.cc

@@ -62,7 +62,9 @@ std::string GetThunkCcFile(Context& ctx, const FileDescriptor& file) {
 
 std::string GetHeaderFile(Context& ctx, const FileDescriptor& file) {
   auto basename = StripProto(file.name());
-  return absl::StrCat(basename, ".proto.h");
+  constexpr absl::string_view kCcGencodeExt = ".pb.h";
+
+  return absl::StrCat(basename, kCcGencodeExt);
 }
 
 std::string RawMapThunk(Context& ctx, const Descriptor& msg,