Fix another incorrect data type choice from commit dc2123400.

Fix another incorrect data type choice from commit dc2123400.

add_file_to_manifest declared its mtime argument as pg_time_t,
apparently on the principle that copy-and-paste from the backend
is fine.  However, the callers are passing struct stat's st_mtime
field which is plain time_t, and add_file_to_manifest itself is
passing the value to gmtime(3) which expects plain time_t,
so the whole thing would not work at all on any platform where
those types are different.  Fortunately we can just switch this
variable to time_t.

Per warnings from assorted buildfarm members.