.gitignore: ignore y.output

Change-Id: I95b72b6be39fbb923b5f0743d17d7f8bd3ee3814
Reviewed-on: https://go-review.googlesource.com/10860
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.gitignore

@@ -24,7 +24,7 @@ doc/articles/wiki/*.bin
 misc/cgo/life/run.out
 misc/cgo/stdio/run.out
 misc/cgo/testso/main
-src/cmd/*/y.output
+src/cmd/**/y.output
 src/cmd/cgo/zdefaultcc.go
 src/cmd/go/zdefaultcc.go
 src/cmd/internal/obj/zbootstrap.go