Align the message buffer to eight bytes.

The 'struct ptp_message" includes a 64 bit integer field, ts.pdu.sec,
and this must be aligned to an eight byte boundary for armv5 machines.
Although the compiler puts the field at the right offset, we spoil this
by packing the struct with 20 bytes of head room. This patch fixes the
issue by realigning the message buffer.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>