Add PGDLLIMPORT to enable_hashagg

Hey all,

There was already a discussion and commit for adding PGDLLIMPORT to some
variables which enables extensions to use them on Windows builds. For
reference, the previous thread:"Add PGDLLIMPORT lines to some variables".

I would like to add PGDLLIMPORT to enable_hashagg for the same reason. I'm
adding a very simple patch. Please let me know if I'm missing anything
given that this is my first patch submission.

Best,
Metin