Fix stand/ build after r339671.

ffs_subr.c requires calculate_crc32c() from libkern. Unfortunately we
cannot just add libkern/crc32.c to libstand because crc32.o is already
compiled from contrib/zlib/crc32.c. Use the include trick to rename
the source.

Note that libstand also provides crc32.c which seems to be unused.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D17677