rofi-wayland: add meta.mainProgram

pkgs/applications/misc/rofi/wayland.nix

···
     description = "Window switcher, run dialog and dmenu replacement for Wayland";
     homepage = "https://github.com/lbonn/rofi";
     license = licenses.mit;
+    mainProgram = "rofi";
     maintainers = with maintainers; [ bew ];
     platforms = with platforms; linux;
   };