libxfont2: Document more specific BSD-4-Clause-UC

libxfont2: Document more specific BSD-4-Clause-UC

COPYING in libxfont2 upstream contains only one BSD-4-Clause text variant,
which is explicitly covered by the copyright addendum by University of
California. Change the SPDX license reference to BSD-4-Clause-UC.

License-Update: reference can be more specific

Signed-off-by: Bastian Germann <bage@debian.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>