testptp: promote 'perout' variable to int64_t

Since 'perout' holds the nanosecond value of the signal's period, it
should be a 64-bit value. Current assumption is that it cannot be larger
than 1 second.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>