AFAIK you can't link to a non-gpl compatible library from a gpl application. You need to find some GPL/LGPL alternative. Alternatively you make make an external library which links to that closed source library and make using it from your application an option -- meaning that your app has to be able to run without it (for example by using some GPL'd alternative).

Another option could be to dynamically load the closed source library using QLibrary and resolve its symbols manually. I don't know what does GPL consider such a situation -- you're not linking with the violating lib, but you still depend on it (unless you make it an option as stated before).