kheaders: use standard naming for the temporary directory

If the kheaders archive generation is interrupted then this directory
may be left on disk and not ignored by git.
By using the standard naming schema for temporary files and directories
the default .gitignore and "make clean" rules will apply.

Suggested-by: Nicolas Schier <nicolas@fjasle.eu>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>