Add C# support std::unique_ptr inputs

Based on Java implementation.

Examples/test-suite/cpp11_std_unique_ptr.i

@@ -53,7 +53,7 @@ struct KlassInheritance : virtual Klass {
   }
 };
 
-#if defined(SWIGJAVA)
+#if defined(SWIGJAVA) || defined (SWIGCSHARP)
 std::string takeKlassUniquePtr(std::unique_ptr<Klass> k) {
   return std::string(k->getLabel());
 }