dnscrypt-proxy: enable additional plugins

Adding ldns builds additional plugins for rewriting DNS queries,
such as blocking and forwarding.

For example, to use a custom domain blocklist, you can do
```nix
let
  myBlockListFile = writeText "blocked-domains" ''
    *.example.com
  '';
in
{
  services.dnscrypt-proxy.extraArgs = [
    "-X libdcplugin_example_ldns_blocking.so,--domains=${myBlockListFile}"
  ];
}
```

pkgs/tools/networking/dnscrypt-proxy/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, pkgconfig, libsodium, systemd }:
+{ stdenv, fetchurl, pkgconfig, libsodium, ldns, openssl, systemd }:
 
 with stdenv.lib;
 
@@ -15,7 +15,8 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ pkgconfig ];
 
-  buildInputs = [ libsodium ] ++ optional stdenv.isLinux systemd;
+  # <ldns/ldns.h> depends on <openssl/ssl.h>
+  buildInputs = [ libsodium openssl.dev ldns ] ++ optional stdenv.isLinux systemd;
 
   postInstall = ''
     # Previous versions required libtool files to load plugins; they are