gem-config: add curses (#45068)

pkgs/development/ruby-modules/gem-config/default.nix

···
     buildInputs = [ curl ];
   };
 
+  curses = attrs: {
+    buildInputs = [ ncurses ];
+  };
+
   dep-selector-libgecode = attrs: {
     USE_SYSTEM_GECODE = true;
     postInstall = ''