stdio.h: don't expose rsize_t unless __EXT1_VISIBLE

This is how the other typedefs for rsize_t handle it (in <stddef.h>,
<stdlib.h>, and <string.h>). In particular, we shouldn't have any
rsize_t if a C environment earlier C11 was requested.

This reapplies b35f0aa4952c, chasing c25e55bcf80b, fixing ports build
failures following b35f0aa4952c without c25e55bcf80b.

PR: 281768
Sponsored by: Tarsnap Backup Inc.
Signed-off-by: Graham Percival