"qemu/typedefs.h" is enough for include/block/write-threshold.h header
with forward declaration of BlockDriverState. Also drop extra includes
from block/write-threshold.c and tests/unit/test-write-threshold.c

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210506090621.11848-9-vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>