Do not install headers for private classes

When merging the accessibility plugin into the widgets library, the
headers were just moved. They should have gotten the _p at that time.

Task-number: QTBUG-47569
Change-Id: I0a2290dae3a8187596e9d7541ccf69beeb603296
Reviewed-by: Dimitar Dobrev
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>