maint: indent with spaces, not TABs

maint: indent with spaces, not TABs

I need to get in the habit of running gnulib's "make check".
Both of these would have been caught.
* m4/largefile.m4: Indent with spaces, not TABs.
* lib/parse-datetime.y (iso_8601_time): Likewise.
Spotted by Pádraig Brady.