You are presumably using the Nokia Qt SDK because you want to use Qt, and as such you should really avoid making calls to Avkon when not absolutely necessary. How about using QMessageBox?
Nokia Qt SDK has some Symbian libraries so that you can call OS specific functionality, but not much of the deprecated Avkon framework which Qt is intended to replace. As you can see below, there is no trace of the library you seek (unless you have added content to the SDK):
If you want to add more libraries to the SDK (NOT RECOMMENDED) you can take them from the S60 5th Edition SDK. But please note that once you have done so, there should no longer exist any presumption that the applications built with the Nokia Qt SDK are compatible with S60 3.x devices. The resulting app may work on those devices, but thorough testing is needed before making such claim.
c:\NokiaQtSDK\Symbian\SDK\epoc32\release\armv5\lib>dir /b akn*.*