linux-postmarketos-imx-ereader: enable zstd firmware compression support
Needed for https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/60282 as kobo-clara2e uses linux-firmware.
Needed for https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/60282 as kobo-clara2e uses linux-firmware.