Clean up local redeclarations of variables with DLLIMPORT, per report

contrib/adminpack/adminpack.c

contrib/pg_buffercache/pg_buffercache_pages.c

contrib/pg_freespacemap/pg_freespacemap.c

src/backend/catalog/pg_proc.c

src/backend/utils/misc/guc.c

src/include/miscadmin.h

src/include/postmaster/syslogger.h

src/include/storage/buf_internals.h

src/include/storage/freespace.h

src/include/utils/guc.h

src/pl/plperl/plperl.c

src/pl/plpgsql/src/pl_handler.c