project.config: explicitly grant tag creation to golang-gob-owners

I didn't think this would be necessary, but perhaps Administrate Server is more limited than I thought it is.

Change-Id: I234aeb39c4a7a6a13f62a1aaaf095ee4303f9785
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/488395
Reviewed-by: Michael Pratt <mpratt@google.com>