Require kdecoration >= 5.18.0

Summary:
To successfully build kwin needs 13e6459a3baeebe6a7b594efd995b88f6dd7524e
in kdecoration. Otherwise it fails with: "previewclient.h:93:11: error:
'QSize KDecoration2::Preview::PreviewClient::size() const' marked
'override', but does not override QSize size() const override;"
So this is not a new requirement but just an update to the build system
to reflect the actual requirements.

Test Plan: Builds fine with the newly required version

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27242