cmd/vet: don't warn on expected space in XML tag

cmd/vet: don't warn on expected space in XML tag

The change in https://golang.org/cl/43295 added warning about spaces in
struct tags. However, in XML tags it is expected that there will be a
space between the namespace and the local name.

Change-Id: Ic31c3bdae30797f406f25c737b83bbe2de1ed1db
Reviewed-on: https://go-review.googlesource.com/62570
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>