Make Qt5::X11Extras really optional

Summary:
Otherwise may fail with:
```
CMake Warning at CMakeLists.txt:32 (find_package):
  Found package configuration file:

/Users/kfunk/Qt5.6.2/5.6/clang_64/lib/cmake/Qt5/Qt5Config.cmake

  but it set Qt5_FOUND to FALSE so package "Qt5" is considered to be NOT
  FOUND.  Reason given by package:

...

  Failed to find Qt5 component "X11Extras" config file at
  "/Users/kfunk/Qt5.6.2/5.6/clang_64/lib/cmake/Qt5X11Extras/Qt5X11ExtrasConfig.cmake" CMake Error at /usr/local/Cellar/cmake/3.6.2/share/cmake/Modules/FeatureSummary.cmake:556 (message):
  feature_summary() Error: REQUIRED package(s) are missing, aborting CMake
  run.
```

Reviewers: rjvbb, sitter, davidedmundson

Reviewed By: sitter, davidedmundson

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8307