[minor] Fix compilation warning

[dentiny]

I get the following compilation warning on build.

/home/vscode/duckdb/build/debug/_deps/httpfs_extension_fc-src/src/httpfs_httplib_client.cpp: In member function ‘virtual duckdb::unique_ptr<duckdb::HTTPResponse> duckdb::HTTPFSClient::Post(duckdb::PostRequestInfo&)’:
/home/vscode/duckdb/build/debug/_deps/httpfs_extension_fc-src/src/httpfs_httplib_client.cpp:118:33: warning: moving a local object in a return statement prevents copy elision [-Wpessimizing-move]
  118 |                 return std::move(transformed_req);
      |                        ~~~~~~~~~^~~~~~~~~~~~~~~~~

[carlopi]

Thanks!