amd64/pmap: include opt_kstack_pages.h

This fixes an early KASAN initialization panic in
pmap_san_enter_early_alloc_4k, when a non-default value is specified for
KSTACK_PAGES in the build config file.

Sponsored by: Juniper Networks, Inc.
MFC after: 7 days
Reviewed by: des, markj
Differential Revision: https://reviews.freebsd.org/D51709