emulators/virtualbox-ose-kmod{,-70,-71,-legacy}: Fix on 15-CURRENT: linker_load_file: /boot/modules/vboxnetflt.ko - unsupported file type

After commit
https://github.com/freebsd/freebsd-src/commit/93b4a54456589e78dcd37f2db8333baff3f3f0b2

kernel: linker_load_file: /boot/modules/vboxnetflt.ko - unsupported file type
kernel: KLD vboxnetflt.ko: depends on netgraph - not available or version mismatch

Thanks to Benjamin Close <Benjamin.Close@clearchain.com> and
rkoberman@gmail.com for researching the issue and identifying its cause.

PR: 286204 286717
MFH: 2025Q2