go/build: fix field mismatch in unkeyed struct literal

Fixes #9409

Change-Id: I2404cd8bf3ebb07f4b6a2b3e1d58ab69b9f1e8d8
Reviewed-on: https://go-review.googlesource.com/2040
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>