aacenc: Mark psy_3gpp_window() as av_unused.

It is intentionally left in to allow adding 3GPP-style windowing in the future.
Marking it av_unused silences an annoying unused function warning.

libavcodec/aacpsy.c

@@ -377,9 +377,10 @@ static const uint8_t window_grouping[9] = {
  * Tell encoder which window types to use.
  * @see 3GPP TS26.403 5.4.1 "Blockswitching"
  */
-static FFPsyWindowInfo psy_3gpp_window(FFPsyContext *ctx,
-                                       const int16_t *audio, const int16_t *la,
-                                       int channel, int prev_type)
+static av_unused FFPsyWindowInfo psy_3gpp_window(FFPsyContext *ctx,
+                                                 const int16_t *audio,
+                                                 const int16_t *la,
+                                                 int channel, int prev_type)
 {
     int i, j;
     int br               = ctx->avctx->bit_rate / ctx->avctx->channels;