Back-patch addition of PGDLLIMPORT to un-break Windows BF animals.

Back-patch addition of PGDLLIMPORT to un-break Windows BF animals.

drongo has been failing build in the v14 branch since 67d318e70:

  regress.obj : error LNK2001: unresolved external symbol PGLZ_strategy_always [C:\\prog\\bf\\root\\REL_14_STABLE\\pgsql.build\\regress.vcxproj]

This is evidently because PGLZ_strategy_always was not marked
PGDLLIMPORT in that branch.  Back-patch the relevant part of
8ec569479.  (Maybe we should back-patch more of it, but I'm
content to just touch this one file.)