Add copyright header to legacy.c

Change-Id: I14d901112b810dfad1764188d0d01cf50293ba9c
Reviewed-on: http://review.couchbase.org/44293
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>