Make zil_async_to_sync visible to platform code

FreeBSD's zvol platform code requires access to the
zil_async_to_sync() function.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9440