Don't remove -framework on Mac.

"LIBS -= -framework QtSerialPort" removes all occurrences of -framework
which breaks the build if you use a framework together with QtSerialPort.
To avoid this we skip the call to qtAddLibrary under Mac.

Task-number: QTBUG-37989
Change-Id: I53b71ad433e93c8c59d17c526ef2424db47bca15
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>