crypto/chacha20poly1305/internal/chacha20: add missing copyright header

The file was originally added in 2016.

Change-Id: I1b5c01400fb73e83f39c086ea1235a948d27308d
Reviewed-on: https://go-review.googlesource.com/35874
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed