pgsql: Back-patch addition of PGDLLIMPORT to un-break Windows BF animal

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.)

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f76685b250c12f7449a3938b9a27befa812057c4

Modified Files
--------------
src/include/common/pg_lzcompress.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)