cgen: fix error of reference struct str() (#12061)

vlib/v/tests/str_reference_struct_test.v

@@ -0,0 +1,12 @@
+struct Example {
+}
+
+fn test_str_reference_struct() {
+	c1 := Example{}
+	println((&c1).str())
+	assert (&c1).str() == '&Example{}'
+
+	c2 := &Example{}
+	println(c2.str())
+	assert c2.str() == '&Example{}'
+}