meson: unbreak sysctl.h detection on BSDs
<sys/sysctl.h> is not standalone on FreeBSD and OpenBSD. According to manpage either <sys/types.h> or <sys/param.h> is necessary but style discourages using both. CC @zeising
Currently, only affects swrast (gallium) which falls back to sysconf.
https://man.freebsd.org/sysctl/3
https://man.openbsd.org/sysctl.2