correct Settings.isMobile usage

Summary:
Settings.isMobile is in the Kirigami import.

Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>

Test Plan: Check that the error in the console "Settings.isMobile" is not defined is gone.

Reviewers: mart

Reviewed By: mart

Subscribers: davidedmundson, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D8291