qjson: allow building with Qt5
Since commit 18714011b231fc326c96782134fbea778ab6a94d ("qjson: bump
version"), qjson can be built either against Qt4 or Qt5. This commit
therefore allows this package to be selected when Qt5 is enabled.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>