Add missing PGDLLEXPORT markings in contrib/pg_prewarm.

Add missing PGDLLEXPORT markings in contrib/pg_prewarm.

After commit 089480c07, it's necessary for background worker entry
points to be marked PGDLLEXPORT, else they aren't findable by
LookupBackgroundWorkerFunction().  Since pg_prewarm lacks any
regression tests, it's not surprising its worker entry points were
overlooked.  (A quick search turned up no other such oversights.)

I added some documentation pointing out the need for this, too.

Robins Tharakan and Tom Lane

CAEP4nAzndnQv3-1QKb=D-hLoK3Rko12HHMFHHtdj2GQAUXO3gw@mail.gmail.com