Blackfin: Rename DES PC2() symbol to avoid collision

Rename the PC2() symbol in the generic DES crypto module to be prefixed with
DES_ to avoid collision with arch code (Blackfin in this case).

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>