qt: Rework how we find the Qt QPA header

Instead of querying the Qt include path from the dependency or from
qmake, rely on the qt5qml_dep to set the include path to QtGui
correctly, and look for the header inside the private includedir.

Then we can use that path to include the header directly.

Reported in gst-plugins-base#780 (comment 548092)

Part-of: <!640>