bhyve: add QEMU_FWCFG_INDEX_NB_CPUS item

Requested-by: OVMF (qemu)
Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39316

(cherry picked from commit e46be58cca6bd180dadfb36e98a673ae6d8d2cd1)

bhyve: add missing include to qemu_fwcfg.c

Fixes: e46be58cca6bd180dadfb36e98a673ae6d8d2cd1 ("bhyve: add QEMU_FWCFG_INDEX_NB_CPUS item")
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 76fa62b5232e67ef10476cf1329aaceb9cbc2ff5)