emulators/virtualbox-ose-kmod: Fix build with LLVM 15

Starting with clang 15, the -Wint-conversion warning diagnostic for
implicit int <-> pointer conversions defaults to an error. Use a cast to
fix such an error here.

See also: https://lists.freebsd.org/archives/freebsd-emulation/2023-February/000431.html

https://lists.freebsd.org/archives/freebsd-emulation/2023-February/000460.html

PR: 269721
Reported by: fbl@aoek.com, ohartmann@walstatt.org, rkoberman@gmail.com
Differential Revision: https://reviews.freebsd.org/D38906
Sponsored by: The FreeBSD Foundation