syncthingtray: set mainProgram (#382867)

Author: drupol

pkgs/applications/misc/syncthingtray/default.nix

@@ -120,5 +120,6 @@ stdenv.mkDerivation (finalAttrs: {
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ doronbehar ];
     platforms = platforms.linux ++ platforms.darwin;
+    mainProgram = "syncthingtray";
   };
 })