Removed Qt4 private headers test since it's hard to get Qt4 private
headers installed on CI.
Signed-off-by: Alexis Jeandet <alexis.jeandet@member.fsf.org>
This commit adds private_headers option in dependency method which tells
QtDependency to add private headers include path to build flags.
Since there is no easy way to do this with pkg-config only qmake method
supports this, so with private_headers set qmake will always be used.
Signed-off-by: Alexis Jeandet <alexis.jeandet@member.fsf.org>