xorg-minimal-fonts: Really fix determinism

My previous fix wasn't correct as the file timestamps do vary by git checkout
or modification time and aren't correct here. Instead use a specific
date/time for the files to be deterministic.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 725a30a3)
Signed-off-by: Steve Sakoman <steve@sakoman.com>