build failure -amd64, fix.

[B Harder <brad.harder%gmail.com@localhost>]

--- /usr/src/usr.bin/cksum/crc.c        2014-10-29 11:27:59.000000000 -0700
+++ /usr/src/usr.bin/cksum/crc.bch      2014-10-29 11:27:52.000000000 -0700
@@ -115,7 +115,7 @@
 crc(int fd, u_int32_t *cval, off_t *clen)
 {
        u_char *p;
-       int nr;
+       ssize_t nr;
        u_int32_t thecrc;
        off_t len;
        u_char buf[16 * 1024];