libspl: declare aok extern in header

Rather than defining a new instance of 'aok' in every compilation
unit which includes this header, there is a single instance
defined in zone.c, and the header now only declares an extern.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Signed-off-by: Nick Black <dankamongmen@gmail.com>
Closes #9752