ng_source(4): correction after the change r340617

tv_usec has "long" type for all architecture in FreeBSD
and follows LP64. However, this is not true for tv_sec
that has "time_t" type.

Since r320347 that changed time_t from 32 to 64 bit integer
for 32 bit version of powerpc architecture, we have only single
i386 architecture having 32 bit time_t type.

Submitted by: jhb
MFC after: 1 week.