[SCSI] make scsi_wait_scan always modular

Currently scsi_wait_scan is only built modular if SCSI is modular.
However, it's perfectly possible for a built in SCSI still to have
modular drivers and thus need scsi_wait_scan as a module.  Therefore,
scsi_wait_scan should always be built as a module (unless the kernel
doesn't support modules).

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