Rename private header qdebug_p.h to qxmldebug_p.h.

Avoid conflicts with the newly introduced qdebug_p.h
of QtCore.

Change-Id: I04b83e159900d9d36fa96ae5112eb4cdf75c0551
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>