xorg-fonts-minimal: Fix reproducibility

When installing the font files, preserve their timestamp rather than using
the current time which fixes reproducibility issues.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit a0c4f4b0)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>