proto: fix compilation failure in tests

Change-Id: I61a84dffcf4ccc9f81dc8b9f932c315713e81142
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/418674
Reviewed-by: Lasse Folger <[email protected]>

Author: neild

proto/equal_test.go

@@ -632,11 +632,11 @@ func makeNested(depth int) *testpb.TestAllTypes {
 	if depth <= 0 {
 		return nil
 	}
-	return testpb.TestAllTypes_builder{
-		OptionalNestedMessage: testpb.TestAllTypes_NestedMessage_builder{
+	return &testpb.TestAllTypes{
+		OptionalNestedMessage: &testpb.TestAllTypes_NestedMessage{
 			Corecursive: makeNested(depth - 1),
-		}.Build(),
-	}.Build()
+		},
+	}
 }
 
 func BenchmarkEqualWithDeeplyNestedEqual(b *testing.B) {