scripts/extract-ikconfig: add zstd compression support

Add extract-ikconfig support for kernel images compressed with zstd.

Signed-off-by: Thitat Auareesuksakul <thitat@flux.ci>
Tested-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>