lavfi/volume: enable runtime change flag

 enable runtime change flag.
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavfilter/af_volume.c

@@ -62,10 +62,11 @@ static const char *const var_names[] = {
 #define OFFSET(x) offsetof(VolumeContext, x)
 #define A AV_OPT_FLAG_AUDIO_PARAM
 #define F AV_OPT_FLAG_FILTERING_PARAM
+#define T AV_OPT_FLAG_RUNTIME_PARAM
 
 static const AVOption volume_options[] = {
     { "volume", "set volume adjustment expression",
-            OFFSET(volume_expr), AV_OPT_TYPE_STRING, { .str = "1.0" }, .flags = A|F },
+            OFFSET(volume_expr), AV_OPT_TYPE_STRING, { .str = "1.0" }, .flags = A|F|T },
     { "precision", "select mathematical precision",
             OFFSET(precision), AV_OPT_TYPE_INT, { .i64 = PRECISION_FLOAT }, PRECISION_FIXED, PRECISION_DOUBLE, A|F, "precision" },
         { "fixed",  "select 8-bit fixed-point",     0, AV_OPT_TYPE_CONST, { .i64 = PRECISION_FIXED  }, INT_MIN, INT_MAX, A|F, "precision" },