Add missing UIKit import to UIWindow extension

[jjochen]

fixes #292

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (163ac47) to head (cd40e2e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #293 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 11 11 Lines 952 952 ========================================= Hits 952 952 ```

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