lavfi: rename af_volume to af_volume_stefano

This matches the naming style of the new af_volume_justin filter.

Note, Yes i would too prefer having one filter instead of 2.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/Makefile

@@ -71,7 +71,7 @@ OBJS-$(CONFIG_JOIN_FILTER)                   += af_join.o
 OBJS-$(CONFIG_PAN_FILTER)                    += af_pan.o
 OBJS-$(CONFIG_RESAMPLE_FILTER)               += af_resample.o
 OBJS-$(CONFIG_SILENCEDETECT_FILTER)          += af_silencedetect.o
-OBJS-$(CONFIG_VOLUME_FILTER)                 += af_volume.o
+OBJS-$(CONFIG_VOLUME_FILTER)                 += af_volume_stefano.o
 OBJS-$(CONFIG_VOLUME_JUSTIN_FILTER)          += af_volume_justin.o
 OBJS-$(CONFIG_VOLUMEDETECT_FILTER)           += af_volumedetect.o