gtklock: set mainProgram

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

pkgs/tools/wayland/gtklock/default.nix

···
     license = licenses.gpl3;
     maintainers = with maintainers; [ dit7ya ];
     platforms = platforms.linux;
+    mainProgram = "gtklock";
   };
 }