Fix headers inclusion in xf86drmMode.c

Add sys/sysctl.h to get sysctlbyname declaration on kFreeBSD

Updated by Thorsten “mirabilos” Glaser <t.glaser@tarent.de>
to add autoconf check and only include <sys/sysctl.h> if it
is detected by configure as it’s unusable on Linux/x32 (and
others, e.g. other new architectures).

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>