Import missing enum declaration in pci_host_generic header file

Other files including pci_host_generic.h failed to compile
due to missing declaration of enum pci_id_type.

Obtained from: Semihalf
Submitted by: Michal Stanek <mst@semihalf.com>
Sponsored by: Annapurna Labs
Reviewed by: wma
Differential Revision: https://reviews.freebsd.org/D7561