libata.h: add stub for ata_sas_port_resume

Fix build error when CONFIG_PM is not enabled by adding a stub
function in <linux/libata.h>.

drivers/scsi/libsas/sas_ata.c: In function 'sas_resume_sata':
drivers/scsi/libsas/sas_ata.c:756:3: error: implicit declaration of function 'ata_sas_port_resume' [-Werror=implicit-function-declaration]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>