Request matching version of plasma-workspace libs

Summary:
We never remember to manually update it given the tight plasma coupling.

We use ${PROJECT_VERSION} when finding Breeze so the concept clearly
works and it shouldn't have any more negative impact.

Test Plan: Compiles

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: bcooksley, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23569