emulators/virtualbox-ose: fix build emulators/virtualbox-ose-additions{,-nox11} on main: error: unknown type name 'uma_zone_t'

In file included from /wrkdirs/usr/ports/emulators/virtualbox-ose-additions/work/VirtualBox-6.1.50/src/VBox/Additions/freebsd/vboxvfs/vboxvfs_prov.c:42:
/wrkdirs/usr/ports/emulators/virtualbox-ose-additions/work/VirtualBox-6.1.50/src/VBox/Additions/freebsd/vboxvfs/vboxvfs.h:209:2: error: unknown type name 'uma_zone_t'

209 |         uma_zone_t      sf_node_pool;
    |         ^

Reported by: pkg-fallout
MFH: 2024Q3