.gitignore: ignore src/cmd/dist/dist

In case of a failed/cancelled build, src/cmd/dist/dist might be left in
place.

Change-Id: Id81b5d663476a880101a2eed54fa051c40b0b0bc
Reviewed-on: https://go-review.googlesource.com/74150Reviewed-by: Ian Lance Taylor <iant@golang.org>

.gitignore

@@ -31,6 +31,7 @@ _testmain.go
 /pkg/
 /src/*.*/
 /src/cmd/cgo/zdefaultcc.go
+/src/cmd/dist/dist
 /src/cmd/go/internal/cfg/zdefaultcc.go
 /src/cmd/go/internal/cfg/zosarch.go
 /src/cmd/internal/objabi/zbootstrap.go