message: sv2017: example p541: add missing module wrap
diff: class C1;
  rand integer x;
  endclass
  +
  class C2;
  rand integer y;
  endclass
  +
  +module top;
  initial begin
  C1 c1 = new();
  C2 c2 = new();
  @@ -12,3 +15,4 @@ void'(c1.randomize());
  // z = $random;
  void'(c2.randomize());
  end
  +endmodule
  \ No newline at end of file
  