iperf3: fix compilation to support authentication creds

Service iperf3 defines authorizedUsersFile and rsaPrivateKey to
enable authenticated iperf server. It requires iperf to be compiled
with openssl.

https://nixos.org/nixos/options.html#services.iperf3.authorizedusersfile

pkgs/tools/networking/iperf/3.nix

@@ -9,6 +9,9 @@ stdenv.mkDerivation rec {
   };
 
   buildInputs = [ openssl ];
+  configureFlags = [
+    "--with-openssl=${openssl.dev}"
+  ];
 
   outputs = [ "out" "man" ];