libzstd: also build with LIBZPOOL_CPPFLAGS

libzstd now also allocates its own abd_t, and so has the same issue as
zstream did, so this applies the same workaround: compile it with
ZFS_DEBUG. See 92fca1c2d.

This looks weird, because libzstd doesn't appear to look related to the
ZFS kernel, but there is already a cross-dependency there: zstd needs
zfs_lz4_compress, and zfs needs zfs_zstd_compress (and others), so the
two can never really be separated without more work. Another job for
another time.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Mark Maybee <mmaybee@delphix.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16489