Fix qt5 compile (issue #336) with SliceViewer Fix (issue #335)

[kerautret]

PR Description

As mentioned by @ngophuc it fix #336 and also by the way #335..

Checklist

• Doxygen documentation of the code completed (classes, methods, types, members...).
• Main tool doxygen documentation (following existing documentation of DGtalTools documentation.
• Check if it follows the tools structure described in CONTRIBUTING.md
• New entry in the ChangeLog.md added.
• Update the readme with potentially a screenshot of the tools if it applies.
• No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).

[dcoeurjo]

Thanks, merging