community/knot-resolver: support nghttp2
To enable the doh2 feature, kresd requires the libnghttp2 library. This commit adds a dependency on main/nghttp2 (nghttp2-libs).
The doh2 is the DoH implementation recommended in the official documentation. https://www.knot-resolver.cz/documentation/stable/daemon-bindings-net_tlssrv.html#dns-over-https-doh