PGDLLIMPORT'ify DateStyle and IntervalStyle.

This is needed on Windows to support contrib/postgres_fdw.  Although it's
been broken since last March, we didn't notice until recently because there
were no active buildfarm members that complained about missing PGDLLIMPORT
marking.  Efforts are underway to improve that situation, in support of
which we're delaying fixing some other cases of global variables that
should be marked PGDLLIMPORT.  However, this case affects 9.3, so we
can't wait any longer to fix it.

I chose to mark DateOrder as well, though it's not strictly necessary
for postgres_fdw.