rootfs.py: find .ko.zst kernel modules

With CONFIG_MODULE_COMPRESS_ZSTD enabled, kernel modules will have a
.ko.zst extension. This fixes depmod not being run.

Fixes: 1b696a45 ("rootfs.py: Add check for kernel modules before running depmod")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>