Fix compatibility with old FFmpeg

V4L2 sink used a "url" field format AVFormatContext which has been
introduced in lavf 58.7.100.

Fixes #2382 <https://github.com/Genymobile/scrcpy/issues/2382>

Refs <ea3672b7d6>
Refs <fa8308d3d4>

app/src/v4l2_sink.c

@@ -181,16 +181,16 @@ sc_v4l2_sink_open(struct sc_v4l2_sink *vs) {
     // <https://github.com/FFmpeg/FFmpeg/commit/0694d8702421e7aff1340038559c438b61bb30dd>
     vs->format_ctx->oformat = (AVOutputFormat *) format;
 #ifdef SCRCPY_LAVF_HAS_AVFORMATCONTEXT_URL
-    vs->format_ctx->url = strdup(vs->device_name);
+# define CTX_URL_FIELD url
+#else
+# define CTX_URL_FIELD filename
+#endif
+    vs->format_ctx->CTX_URL_FIELD = strdup(vs->device_name);
     if (!vs->format_ctx->url) {
         LOGE("Could not strdup v4l2 device name");
         goto error_avformat_free_context;
         return false;
     }
-#else
-    strncpy(vs->format_ctx->filename, vs->device_name,
-            sizeof(vs->format_ctx->filename));
-#endif
 
     AVStream *ostream = avformat_new_stream(vs->format_ctx, encoder);
     if (!ostream) {