libata: libahci: declare ahci_shost_attr_group as static

ahci_shost_attr_group is referenced only in drivers/ata/libahci.c.
Declare it as static.

Fixes: c3f69c7f

 ("scsi: ata: Switch to attribute groups")
Cc: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>