apps/nshlib: Don't try to flush output streams if stdio buffered I/O is not supported.

nshlib/nsh_system.c

@@ -84,7 +84,9 @@ int nsh_system(int argc, char *argv[])
       /* Parse process the command */
 
       (void)nsh_parse(vtbl, argv[1]);
+#if CONFIG_NFILE_STREAMS > 0
       fflush(pstate->cn_outstream);
+#endif
 
       /* Exit upon return */