Don't check for the existence of private include directories.

Some packagers don't want to install the private headers.

Check the existence of private headers only if the 'Private' component
is specified when finding the package.

Task-number: QTBUG-32466
Change-Id: I1fdbfb25e8ce485cd051564b937f766b2733741a
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>