Fix <sys/_types.h> issues with <stddef.h>

Fix <sys/_types.h> issues with <stddef.h>

A commit from 2016 tried to address this GCC provided <stddef.h> issue

    #if (defined (__FreeBSD__) && (__FreeBSD__ >= 5)) \
      || defined(__DragonFly__) \
      || defined(__FreeBSD_kernel__)
    /* __size_t is a typedef on FreeBSD 5, must not trash it. */
    #elif defined (__VMS__)
    /* __size_t is also a typedef on VMS.  */
    #else
    #define __size_t
    #endif

with an include of <stddef.h> before <sys/_types.h> in <sys/types.h>.
Is is not robust enough.  Do the include of <stddef.h> in <sys/_types.h>
directly and request only the necessary types.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>