linuxkpi: Move pci_alloc_irq_vectors to .c file

pci_alloc_irq_vectors encodes the size of struct msix_entry
into its code. Move from .h to .c to move this knowledge from
client modules to linuxkpi module.

Sponsored by: Netflix
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D34774

(cherry picked from commit 36b5c440028b44b22cfc0596125f575ca513656f)