Skip qt4support when generating documentation.

Fix warnings:
qtserialport/src/serialport/qt4support/src/qlockfile.cpp:57: warning: Overrides a previous doc
qtbase/src/corelib/io/qlockfile.h:53: warning: (The previous doc is here)

Change-Id: I75c93fd6dbd9bebfa506b2f606aeb8511a892305
Reviewed-by: Laszlo Papp <lpapp@kde.org>