driver: add a space before +build in build tag comment

Add a space before a build tag comment so it corresponds to the format
documented at https://golang.org/pkg/go/build/.

Change-Id: Ic3225a76ca2ac1ce7de1dca7a1bbeaa438929a5a
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/193797
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed