So errnoErr can be used in other packages.
This is something I missed when I sent CL 28990.

Fixes #17539

Change-Id: I8ee3b79c4d70ca1e5b29e5b40024f7ae9a86061e
Reviewed-on: https://go-review.googlesource.com/29690Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>