wlogout: set mainProgram

Signed-off-by: Sefa Eyeoglu <[e-mail]>

pkgs/tools/wayland/wlogout/default.nix

···
     license = licenses.mit;
     maintainers = with maintainers; [ AndersonTorres ];
     platforms = platforms.linux;
+    mainProgram = "wlogout";
   };
 }
 # TODO: shell completions