[R] Fix CopyToR() generated for struct in namespace

Fixes https://sourceforge.net/p/swig/bugs/1147/

Examples/test-suite/namespace_struct.i

@@ -0,0 +1,8 @@
+%module namespace_struct
+
+%inline %{
+
+// SWIG/R generated bad code for copyToR for a struct in a namespace in SWIG < 4.1.0
+namespace X { struct Y { int x, y; }; }
+
+%}