Add note about QT_{BEGIN,END}_HEADER removal for Qt 6

This macro usage has been removed from most of the Qt 5 code, so
adding a note to be completely removed in Qt 6.

Change-Id: I19a90db78745f3cacbcbf206e8642c7d7c36e04a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>