nDisplay Regression: Public header DisplayClusterViewport_OverscanSettings.h now includes internal header Math/MarginSet.h, breaking clients' code

On the nDisplay plugin code, CL 44773085 from August 12 introduced a new struct template TMarginSet and changed struct FDisplayClusterViewport_OverscanSettings to use it. However, the latter is defined in public header DisplayClusterViewport_OverscanSettings.h, while the former is in a private header Math/MarginSet.h. This broke compilation for licensee-made clients of the plugin that were already using FDisplayClusterViewport_OverscanSettings before, since the private header dependency is not visible to them.

1. Make a new plugin module that depends on the "DisplayCluster" module from the "nDisplay" plugin
2. #include "Render/Viewport/Containers/DisplayClusterViewport_OverscanSettings.h" on any file
3. Compile