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