Summary: Fixed Linux-specific Qt plugin path

Summary: Looked at Online Qt 5.6 and 5.10 as well as Fedora's directory layout, fix is necessary for all of these

Test Plan: mitkPackageTest on Linux

Reviewers: kislinsk

Reviewed By: kislinsk

Maniphest Tasks: T24609

Differential Revision: https://phabricator.mitk.org/D139