.gitattributes: prevent all magic line ending changes

Fixes #9281

Change-Id: Id060e9cbad086c994ec74b240f5c61e971e5690b
Reviewed-on: https://go-review.googlesource.com/1490
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>