Don't define WebEnginePartCookieJar::filterCookie for old versions of Qt

Summary:
for versions of Qt before 5.11, WebEnginePartCookieJar::filterCookie
can't be defined because it's argument is of a type introduced in Qt 5.11.0

Test Plan: I couldn't test whether this actually builds on older versions of Qt because my systems all have Qt 5.11

Reviewers: dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D14704