system/trace: force flush the output stream

Signed-off-by: chao.an <anchao@xiaomi.com>

system/trace/trace_dump.c

@@ -653,6 +653,8 @@ static int trace_dump_one(FAR FILE *out,
         break;
     }
 
+  fflush(out);
+
   /* Return the length of the processed note */
 
   return note->nc_length;