[SCSI] fix scsi_wait_scan build problem

The #ifdef MODULE around the export of scsi_complete_async_scans()
which is the API the scsi_wait_scan module uses is incorrect and
causes the symbol to be undefined in certain circumstances leading to
a build failure.  Remove the defines.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>