scsi: fnic: Remove a useless struct mempool forward declaration

struct mempool doesn't currently exist, and thus also isn't used in
fnic.h, remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250812082808.371119-1-hch@lst.de


Reviewed-by: Karan Tilak Kumar <kartilak@cisco.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>