qjson: allow building with Qt5

Since commit 18714011 ("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>