binder: make symbol 'binder_free_buf' static

Fixes the following sparse warning:

drivers/android/binder.c:3312:1: warning:
 symbol 'binder_free_buf' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -3308,7 +3308,7 @@ err_invalid_target_handle:
  *
  * Cleanup buffer and free it.
  */
-void
+static void
 binder_free_buf(struct binder_proc *proc, struct binder_buffer *buffer)
 {
 	if (buffer->transaction) {