COMP: Fix missing initialization braces warning

Fix missing initialization braces warning.

Fixes:
```
[CTest: warning matched]
/Users/builder/externalModules/Core/Common/test/itkMultiThreaderExceptionsTest.cxx:59:37:
warning: suggest braces around initialization of subobject [-Wmissing-braces]
    largestPossibleRegion.SetSize({ 4 });
                                    ^
                                    {}
```

reported at:
https://open.cdash.org/viewBuildError.php?type=1&buildid=7496967

Modules/Core/Common/test/itkMultiThreaderExceptionsTest.cxx

@@ -56,7 +56,7 @@ class ITK_TEMPLATE_EXPORT DummyImageSource : public ImageSource<TOutputImage>
     TOutputImage * output = nullptr;
     output = this->GetOutput(0);
     typename TOutputImage::RegionType largestPossibleRegion;
-    largestPossibleRegion.SetSize({ 4 });
+    largestPossibleRegion.SetSize({ { 4 } });
     output->SetLargestPossibleRegion(largestPossibleRegion);
   }