Fix file listing for symvers in uki
MR2408 added the symvers file to the UKI images, but a previous MR changed compression types to use xz instead of gz, and added a %compext macro to make sure we get the extension correct. This changes the symvers file from a hardcoded gz extension which is no longer in use, to %compext
Signed-off-by: Justin M. Forbes jforbes@fedoraproject.org