generic/754: add _require_symlinks

This test tests whether xattrs can be added to a symbolic link,
which requires symlink support, so add _require_symlinks to check
for symlink support.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Friendy Su <friendy.su@sony.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>