wlopm: add meta.mainProgram

pkgs/tools/wayland/wlopm/default.nix

···
   meta = with lib; {
     description = "Simple client implementing zwlr-output-power-management-v1";
     homepage = "https://git.sr.ht/~leon_plickat/wlopm";
+    mainProgram = "wlopm";
     license = licenses.gpl3Only;
     maintainers = with maintainers; [ arjan-s ];
     platforms = platforms.linux;