kheaders: double-quote variables to satisfy shellcheck
Fix the following:
In kernel/gen_kheaders.sh line 48:
-I $XZ -cf $tarfile -C "${tmpdir}/" . > /dev/null
^-^ SC2086 (info): Double quote to prevent globbing and word splitting.
^------^ SC2086 (info): Double quote to prevent globbing and word splitting.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>