include <sys/types.h> in xf86drmMode when the OS is FreeBSD

<sys/types.h> need to be included in xf86drmMode.c for type u_int in
<sys/sysctl.h> (that is included when OS is FreeBSD) to be recognized.

Signed-off-by: Eleni Maria Stea <elene.mst@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>