Fix crash when accessibility is disabled

With the no-accessibility feature moved to qtgui-config, we don't
always include it where web_content_apapter_client.h is included, which
gives it an inconsistent binary layout.

Solve it by making the optional method always defined as it doesn't
rely on anything from accessibility.

Task-number: QTBUG-61200
Change-Id: I65f34ab2b6763f3166b945e700994bd8d019a835
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>