ring_buffer: rename resereved 'rewind' symbol

This symbol is reserved and usage of reserved symbols violates the
coding guidelines. (MISRA 21.2)

NAME
       fgetpos, fseek, fsetpos, ftell, rewind - reposition a stream

SYNOPSIS
       #include <stdio.h>

       void rewind(FILE *stream);

Signed-off-by: Anas Nashif <anas.nashif@intel.com>