add bin/ to .gitignore.

Change-Id: I5c788d324e56ca88366fb54b67240cebf5dced2c
Reviewed-on: https://go-review.googlesource.com/1171
Reviewed-by: Andrew Gerrand <adg@golang.org>

.gitignore

@@ -48,4 +48,5 @@ goinstall.log
 last-change
 VERSION.cache
 
+bin/
 pkg/