RTEMS uses a considerable part of FreeBSD kernel and user space sources.
These sources are compiled with a __FreeBSD__ define. On 2018-06-26
Gerald Pfeifer changed the GCC provided <stddef.h> so that it includes
<sys/_types.h> if __FreeBSD__ is defined. The Newlib <sys/_types.h>
included <sys/lock.h> which includes <sys/cdefs.h> on RTEMS which
includes <stddef.h>. To get rid of this cyclic dependency move the
optional _flock_t definition to <sys/reent.h>.
Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>