qmake5_paths.bbclass: Use Qt version path for headers

When looking for Qt5 headers we ought to use the Qt5 specific path. In
case the distribution wants to have a flat tree it can set QT_DIR_NAME
and it will still works as expected.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
(cherry picked from commit 2bd5fba34dd350807ac07828cd883d5b6c7ec631)