pdf-quench: add license

pkgs/applications/misc/pdf-quench/default.nix

···
     homepage = https://github.com/linuxerwang/pdf-quench;
     description = "A visual tool for cropping pdf files";
     platforms = platforms.linux;
+    license = licenses.gpl2;
     maintainers = with maintainers; [ flokli ];
   };
 }