uloop: avoid integer overflow in tv_diff

uloop: avoid integer overflow in tv_diff

The tv_diff function can potentially overflow as soon as t2->tv_sec is
larger than 2147483. This is very easily hit in ujail, after only
2147484 seconds of uptime, or 24.85 days.

Improve the behaviour by changing the return type to int64_t.

Fixes: FS#3943
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>