linuxkpi: Add msix_cap to struct pci_dev

At the same time, define PCI_MSIX_FLAGSand PCI_MSIX_FLAGS_ENABLE.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38160

(cherry picked from commit 4152ce2139548e3e3c28c1e224747fa5d10ae656)