Export QQuickLoader so that it can be used outside tests

QQuickLoader is currently exported with Q_AUTOTEST_EXPORT. This means that it’s not possible to use the class (though it’s private) without modifying the export or building Qt with –developer-build option. This also means that the auto tests for QQuickLoader don’t compile with the prebuilt packages.

QQuickLoader should be exported with Q_QUICK_EXPORT (in qtdeclarative\src\quick\items\qquickloader_p.h) to allow people to include the private header and use the class.