Add missing KF5WidgetsAddons and header to fix build

Summary:
This appears to have been implicitly provided by kipi-plugins so far, but
fails to build with kipi-plugins disabled since d20a1533d13cf502ebbab4913f3847254ebc68aa.

Downstream report: https://bugs.gentoo.org/688788

Test Plan: Builds fine with -DCMAKE_DISABLE_FIND_PACKAGE_KF5Kipi=ON

Reviewers: Gwenview, ngraham

Reviewed By: Gwenview, ngraham

Subscribers: nicolasfella

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D22117