knot-resolver: disable libcap-ng

In OpenWrt master (and in upcoming 20.xx), there was added library libcap-ng. When libcap-ng is built before knot-resolver, it is detected by the build system and knot-resolver requires to add libcap-ng to depends. However, libcap-ng can restrict privileges also while Knot Resolver is running under root. Because of that, disable libcap-ng.

Patch is taken from https://gitlab.labs.nic.cz/knot/knot-resolver/-/merge_requests/1012