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>

kernel/gen_kheaders.sh

@@ -45,6 +45,6 @@ rm -f "${tmpdir}.contents.txt"
 # Create archive and try to normalize metadata for reproducibility.
 tar "${timestamp:+--mtime=$timestamp}" \
     --owner=0 --group=0 --sort=name --numeric-owner --mode=u=rw,go=r,a+X \
-    -I $XZ -cf $tarfile -C "${tmpdir}/" . > /dev/null
+    -I "${XZ}" -cf "${tarfile}" -C "${tmpdir}/" . > /dev/null
 
 rm -rf "${tmpdir}"