PCI/ATS: Add inline to pci_prg_resp_pasid_required()

Fix unused function warning when compiled with CONFIG_PCI_PASID
disabled.

Fixes: e5567f5f ("PCI/ATS: Add pci_prg_resp_pasid_required() interface.")
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>