[th/install-libndp-devel] fix installing libndp 1.7.5 devel package for tests

During testing, we can also build NetworkManager form source. We thus need the libndp-devel package as well.

Otherwise, we still end up testing with libndp 1.7.3, which results in a crash (rh #1937721).

Fixes: a79b81f0 ('env: rhel8: install non crashing libndp 1933041')