qt5: disallow the selection of Qt4 and Qt5

As noticed by Arnout, Qt4 and Qt5 cannot be installed together,
because for example the qt.conf installed by one version cannot be
understood by the qmake of another version. We therefore make them
mutually exclusive, which sounds reasonable for the embedded systems
targeted by Buildroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>