include/linux/pci.h: Exclude struct hotplug_slot from KABI

Bugzilla: https://bugzilla.redhat.com/2034338
Depends: https://bugzilla.redhat.com/2024595

Upstream Status: RHEL9 only

Exclude the PCI hotplug_slot struct from KABI calculations. It is only
referenced as a pointer in struct pci.

Signed-off-by: Prarit Bhargava prarit@redhat.com