KCookieJar: fix wrong timezone conversion.

Summary:
Those format strings include the word "GMT" but QDateTime was ignoring
that and setting the timezone to localtime. Set it to UTC so that no
conversion happens in the toUtc() call below.

See debugging details in https://phabricator.kde.org/D14379

Reviewers: stefanocrocco

Reviewed By: stefanocrocco

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14627