This makes K_EXPORT_PLUGIN work with Qt5 again.

In other words, we provide more backwards compatibility than Qt5 does
itself (where Q_EXPORT_PLUGIN just breaks compilation)...

I'm not actually sure why Qt5 does that, it's easy to support to old
macro for compatibility, as proven here (the unittests pass, i.e. the
loading actually works).

This doesn't prevent porting our plugins (and loader, for incremental
migration) to "pure qt5 plugins" though, of course.

,