opensc: locate docbook xsl via filesystem

The opensc formula currently uses the docbook formula to find the
docbook xsl path, which always refers to the newest version. When a
version of docbook is installed, but is not the latest, opensc will fail
to install. This change adopts the same approach as kdelibs in
homebrew-boneyard and globs the appropriate directory to find the path.

Closes #18715.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>