.gitignore: ignore src/go/build/zcgo.go

Change-Id: I6d5db2781d05d6e7e49f5059db1fd84ad6ec328e
Reviewed-on: https://go-review.googlesource.com/19839Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

.gitignore

@@ -28,6 +28,7 @@ src/cmd/**/y.output
 src/cmd/cgo/zdefaultcc.go
 src/cmd/go/zdefaultcc.go
 src/cmd/internal/obj/zbootstrap.go
+src/go/build/zcgo.go
 src/go/doc/headscan
 src/runtime/internal/sys/zversion.go
 src/unicode/maketables