avformat/dashenc: enabling streaming automatically for ldash

There is a little chance that user specified contradicted options
like -streaming 0 -ldash 1, however, it's more likely that user
didn't know or forgot to enable streaming for ldash. So enabling
streaming automatically to make the feature easier to use, similar
like enable FF_MOV_FLAG_FRAGMENT/EMPTY_MOOV/DEFAULT_BASE_MOOF and
so on for FF_MOV_FLAG_CMAF.

libavformat/dashenc.c

@@ -1406,8 +1406,8 @@ static int dash_init(AVFormatContext *s)
     }
 
     if (c->ldash && !c->streaming) {
-        av_log(s, AV_LOG_WARNING, "LDash option will be ignored as streaming is not enabled\n");
-        c->ldash = 0;
+        av_log(s, AV_LOG_WARNING, "Enabling streaming as LDash is enabled\n");
+        c->streaming = 1;
     }
 
     if (c->target_latency && !c->streaming) {