diagnostics: fix gcc-urlifier.cc bootstrap failure [PR112379]

diagnostics: fix gcc-urlifier.cc bootstrap failure [PR112379]

Without the change `./configure --enable-checking=release` bootstrap
fails as:

    gcc/gcc-urlifier.cc:100:1: error: 'get_url_suffix_for_quoted_text()'
        defined but not used [-Werror=unused-function]

This happens because the helper is used only in `ASSERT` macros which
don't always get expanded to executable code.

The fix marks helper function with `ATTRIBUTE_UNUSED`.

gcc/
	PR bootstrap/112379
	* gcc-urlifier.cc (get_url_suffix_for_quoted_text): Mark as
	ATTRIBUTE_UNUSED.