mktempd: silence dd usage

When running a testsuite on FreeBSD, I saw a lot of:

1+ records in
0+1 records out
10 bytes transferred in 0.000063 secs (158875 bytes/sec)

literring the testsuite, and traced it to the fact that we
were silencing one, but not the other, dd usage in our fallback.

* build-aux/mktempd (rand_bytes): Silence dd.

Signed-off-by: Eric Blake <eblake@redhat.com>