Fix linking against Qt5 on MSVC

Fix linking against Qt5 on MSVC

QT_QTMAIN_LIBRARY is no longer automatically set on Qt5, as it is part of
a deprecated way to link against Qt. Since Quassel needs to support both
Qt4 and Qt5, setting the variable to the appropriate value again works
around the issue until such a time that the CMake requirement is increased
to make use of policy CMP0020.

Note that this is the same workaround cmake itself uses when building
cmake-gui for Windows.