superbuild: add new qt5 plugin function

Necessary for adding autio qt5 plugins

* Timothee Chabat: 'optional qt5 plugin dir'
  * 3cb73087 Add new cmake function to add optional Qt5 plugin directory
  * Merge-request: paraview/common-superbuild!551

superbuild @ 7df7c053

-Subproject commit 01946fd755a47721ceccaea36e613a82cf047dd7
+Subproject commit 7df7c053cf5698fa2c18bac9910aaca22acc7059