knot-resolver: add dnstap support

Fortunately the closure increases only by 0.5 MB,
so that sounds suitable to include in the defaults.
Knot Resolver packages for other distros typically do have it.

pkgs/servers/dns/knot-resolver/default.nix

@@ -4,6 +4,7 @@
 # build+runtime deps.
 , knot-dns, luajitPackages, libuv, gnutls, lmdb
 , jemalloc, systemd, libcap_ng, dns-root-data, nghttp2 # optionals, in principle
+, fstrm, protobufc # more optionals
 # test-only deps.
 , cmocka, which, cacert
 , extraFeatures ? false /* catch-all if defaults aren't enough */
@@ -62,9 +63,10 @@ unwrapped = stdenv.mkDerivation rec {
 
   # http://knot-resolver.readthedocs.io/en/latest/build.html#requirements
   buildInputs = [ knot-dns lua.lua libuv gnutls lmdb ]
+    ## the rest are optional dependencies
     ++ optionals stdenv.isLinux [ /*lib*/systemd libcap_ng ]
     ++ [ jemalloc nghttp2 ]
-    ## optional dependencies; TODO: dnstap
+    ++ [ fstrm protobufc ] # dnstap support
     ;
 
   mesonFlags = [