Use FLEXIBLE_ARRAY_MEMBER in a number of other places.

contrib/hstore/hstore_gist.c

contrib/spi/timetravel.c

src/backend/access/heap/syncscan.c

src/backend/commands/async.c

src/backend/libpq/auth.c

src/backend/storage/buffer/freelist.c

src/backend/storage/ipc/sinvaladt.c

src/backend/utils/adt/numeric.c

src/backend/utils/adt/tsgistidx.c

src/backend/utils/adt/tsvector_op.c

src/backend/utils/adt/txid.c

src/backend/utils/fmgr/dfmgr.c

src/backend/utils/sort/logtape.c

src/interfaces/ecpg/ecpglib/extern.h