bitbake.conf: add unzstd in HOSTTOOLS

bitbake.conf: add unzstd in HOSTTOOLS

rpm2cpio.sh can make calls to unzstd to uncompress the RPM payload that
conform the cpio file.

zstd is already part of HOSTTOOLS, as a link to the system installed
zstd.

This patch add unzstd in HOSTOOLS list as a non-optional binary, so is
available to rpm2cpio.sh when it is required.

Signed-off-by: Alberto Planas <aplanas@suse.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>