util/bitset: include util/macro.h

BITSET_FFS(x) macro makes use of ARRAY_SIZE(x) macro which is
defined in util/macro.h. Include it directy to make usage more
straightforward.

Fixes: 692bd4a1 ("util: replace Elements() with ARRAY_SIZE()")
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit efae1279)