proto: use reflect.Ptr for backward compatibility

reflect.Pointer introduced on go1.18. Support 1.17 and earlier.

Change-Id: I62dcdb580a7976068e86df432bc44ee21c2cda81
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/412354
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Joseph Tsai <[email protected]>
Reviewed-by: Lasse Folger <[email protected]>

proto/equal.go

@@ -33,7 +33,7 @@ func Equal(x, y Message) bool {
  if x == nil || y == nil {
  return x == nil && y == nil
  }
- if reflect.TypeOf(x).Kind() == reflect.Pointer && x == y {
+ if reflect.TypeOf(x).Kind() == reflect.Ptr && x == y {
  // Avoid an expensive comparison if both inputs are identical pointers.
  return true
  }