Bug 1774821 - Add a missing include in Windows.

MANUAL PUSH: Trivial bustage fix CLOSED TREE

widget/windows/nsWindow.h

@@ -33,6 +33,7 @@
 #include "mozilla/TimeStamp.h"
 #include "mozilla/webrender/WebRenderTypes.h"
 #include "mozilla/dom/MouseEventBinding.h"
+#include "mozilla/DataMutex.h"
 #include "mozilla/UniquePtr.h"
 #include "nsMargin.h"
 #include "nsRegionFwd.h"