xerrors: add Go version to go.mod

Go 1.12 really wants to add a version (i.e., every time you run "go test").

Change-Id: I1355f7267269e94cdac71f7e979dcacf26d5514a
Reviewed-on: https://go-review.googlesource.com/c/160659
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>