postinst-intercepts/update_font_cache: fix ownership of fontconfig cache

The file ownership of the cache files in /var/cache/fontconfig needs to
be set to root:root otherwise it inherits the user and group id of the
build user.

[YOCTO #7411]

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>