Be more careful about time_t vs. pg_time_t in basebackup.c.

Be more careful about time_t vs. pg_time_t in basebackup.c.

lapwing is complaining that about a call to pg_gmtime, saying that
it "expected 'const pg_time_t *' but argument is of type 'time_t *'".
I at first thought that the problem had someting to do with const,
but Thomas Munro suggested that it might be just because time_t
and pg_time_t are different identifers. lapwing is i686 rather than
x86_64, and pg_time_t is always int64, so that seems like a good
guess.

There is other code that just casts time_t to pg_time_t without
any conversion function, so try that approach here.

Introduced in commit 0d8c9c1210c44b36ec2efcb223a1dfbe897a3661.