kexec-tools: add zstd to package dependencies

kexec-tools: add zstd to package dependencies

Fixed build failure caused by missing libzstd dependency.

Added CONFIG_KEXEC_ZSTD configuration option and libzstd library dependency
declaration, following the same pattern as existing zlib and lzma support.

Fixes: e75218ef4a27 ("kexec-tools: bump to 2.0.32")
Link: https://github.com/openwrt/openwrt/pull/21623#issuecomment-3805115332
Signed-off-by: Roc Lai <laipeng668@qq.com>
Link: https://github.com/openwrt/openwrt/pull/21743
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>