[crypto] Fix debug message

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/x509.c

 			return -EINVAL;
 		}
 		basic->path_len = path_len;
-		DBGC2 ( cert, "X509 %p path length constraint is %u\n",
+		DBGC2 ( cert, "X509 %p path length constraint is %d\n",
 			cert, basic->path_len );
 	}