libstdc++: Avoid '-Wunused-parameter' for 'is_directory' in member function...

libstdc++: Avoid '-Wunused-parameter' for 'is_directory' in member function 'bool std::filesystem::__cxx11::_Dir::do_unlink(bool, std::error_code&) const'

In a newlib configuration:

    ../../../../../source-gcc/libstdc++-v3/src/c++17/fs_dir.cc: In member function ‘bool std::filesystem::__cxx11::_Dir::do_unlink(bool, std::error_code&) const’:
    ../../../../../source-gcc/libstdc++-v3/src/c++17/fs_dir.cc:147:18: error: unused parameter ‘is_directory’ [-Werror=unused-parameter]
      147 |   do_unlink(bool is_directory, error_code& ec) const noexcept
          |             ~~~~~^~~~~~~~~~~~

	libstdc++-v3/
	* src/c++17/fs_dir.cc (do_unlink): Tag 'is_directory' as
	'[[maybe_unused]]'.