dashboard/buildlet: add NetBSD-amd64 to Makefile

Update golang/go#8643

Change-Id: Ie60b87c447180a2a7f2411fcf8fece8035ddfa9d
Reviewed-on: https://go-review.googlesource.com/2602
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

buildlet/Makefile

@@ -16,3 +16,7 @@ buildlet.windows-amd64: buildlet.go
 buildlet.darwin-amd64: buildlet.go
 	GOOS=darwin GOARCH=amd64 go build -o $@ --tags=buildlet
 	cat $@ | (cd ../coordinator/buildongce && go run create.go --write_object=go-builder-data/$@)
+
+buildlet.netbsd-amd64: buildlet.go
+	GOOS=netbsd GOARCH=amd64 go build -o $@ --tags=buildlet
+	cat $@ | (cd ../coordinator/buildongce && go run create.go --write_object=go-builder-data/$@)