[openusd_internal] Adjust top-level namespace

This mitigates Drake build settings, so we don't plan to upstream this patch.

--- pxr/pxr.h.in
+++ pxr/pxr.h.in
@@ -51,6 +51,9 @@ namespace PXR_NS {
 #define PXR_NAMESPACE_CLOSE_SCOPE  }  
 #define PXR_NAMESPACE_USING_DIRECTIVE using namespace PXR_NS;
 
+// Drake addition to paper over the custom namespace.
+namespace pxr = PXR_NS;
+
 #else
 
 #define PXR_NS 
