ast: fix formatting fn variadic of reference param (#11130)

vlib/v/fmt/tests/vargs_reference_param_keep.vv

@@ -0,0 +1,27 @@
+[heap]
+struct Foo {
+	name string
+}
+
+fn agg_stuff(stuffs ...&Foo) []&Foo {
+	stuffs2 := stuffs.clone()
+	return stuffs2
+}
+
+fn arr_stuff(stuffs []&Foo) []&Foo {
+	stuffs2 := stuffs.clone()
+	return stuffs2
+}
+
+fn main() {
+	foo1 := &Foo{'foo'}
+	foo2 := &Foo{'bar'}
+
+	foo11 := agg_stuff(foo1, foo2)
+	println(foo11)
+
+	foo22 := arr_stuff([foo1, foo2])
+	println(foo22)
+
+	assert '$foo11' == '$foo22'
+}