py/mpconfig.h: Add MICROPY_STREAMS_POSIX_API setting.

To filter out even prototypes of mp_stream_posix_*() functions, which
require POSIX types like ssize_t & off_t, which may be not available in
some ports.

py/stream.c

@@ -511,6 +511,7 @@ STATIC mp_obj_t stream_ioctl(size_t n_args, const mp_obj_t *args) {
 }
 MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mp_stream_ioctl_obj, 2, 3, stream_ioctl);
 
+#if MICROPY_STREAMS_POSIX_API
 /*
  * POSIX-like functions
  *
@@ -519,7 +520,6 @@ MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mp_stream_ioctl_obj, 2, 3, stream_ioctl);
  * POSIX-compatible software to work with MicroPython streams.
  */
 
-
 // errno-like variable. If any of the functions below returned with error
 // status, this variable will contain error no.
 int mp_stream_errno;
@@ -568,3 +568,5 @@ int mp_stream_posix_fsync(mp_obj_t stream) {
     }
     return res;
 }
+
+#endif