build: fix when knot-dns headers are on non-standard location

I don't know about all the places with libknot dependency. It's not nice, but I see nothing better than just fixing the particular places whenever they break. (I now tweaked my local setup to use the extra prefix like in opensuse.)

For example, I tried adding dependencies: libknot to libkres_dep but that apparently didn't get propagated to places that have libkres_dep among dependencies :-/

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