sign-file: fix build error in sign-file.c with libressl

The sign-file tool failed to build against libressl. Fix this by extending
the PKCS7 check and thus making sign-file link against libressl without an
error.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>