Add copyright and license information to the crc_stubs.c

Since the original code is based on work by Gary S. Brown

http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/libkern/crc32.c

with the license, "You may use this program, or code or tables extracted
from it, as desired without restriction.", we'll distribute our
derivative work under a similarly permissive ISC license.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>