src/rc/rc-selinux.c: fix use of deprecated security_context_t
src/rc/rc-selinux.c: fix use of deprecated security_context_t

It's only a char* and has been deprecated upstream [0].

[0] https://github.com/SELinuxProject/selinux/commit/7a124ca27581

Fixes: #478

OpenRC/openrc,williamh/openrc,williamh/openrc,williamh/openrc,OpenRC/openrc,OpenRC/openrc