Add missing include in sortedactivitiesmodel to fix build with GCC 7

I did not build-test this, but it's fairly obvious.

Summary: Fixes "error: 'placeholders' is not a namespace-name"

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4464