I've noticed that a lot of people were complaining about not being able to mix Symbian C++ and Qt with Qt Creator. e.g: the orientation lock code would not compile.
I've tried treinio's code from the lock orientation topic, and indeed I was getting linker errors.
The problem in my case was a Qt Creator bug where qmake isn't always run when it should be. I've found that I pretty much have to run qmake manually after modifying the pro file and adding/removing files. In this case it wasn't picking up the new LIBS declaration.