mod_auth_web uses libcurl. Since 83f5929e (component/curl: version up 7.76.0,
including security fixes., 2021-04-07), our curl build depends on zstd.
Apply similar fix that 2f4d8cf8 (component/apache-php: fix missing libzstd, 2021-04-09),
ie. add zstd to rpath