avfilter/af_volume: Also deprecate position enum value

Forgotten in 27f8c9b2.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/af_volume.h

@@ -47,7 +47,9 @@ enum VolumeVarName {
     VAR_NB_CHANNELS,
     VAR_NB_CONSUMED_SAMPLES,
     VAR_NB_SAMPLES,
+#if FF_API_FRAME_PKT
     VAR_POS,
+#endif
     VAR_PTS,
     VAR_SAMPLE_RATE,
     VAR_STARTPTS,