slrn: build slrnpull

As explained in its documentation, also build slrnpull when building
slrn:
https://github.com/jedsoft/slrn/blob/master/doc/slrnpull/SETUP

pkgs/applications/networking/newsreaders/slrn/default.nix

@@ -20,6 +20,7 @@ stdenv.mkDerivation rec {
   configureFlags = [
     "--with-slang=${slang.dev}"
     "--with-ssl=${openssl.dev}"
+    "--with-slrnpull"
   ];
 
   buildInputs = [ slang ncurses openssl ];