lavf: use designated initialisers for all (de)muxers.

It's more readable and less prone to breakage.

libavformat/siff.c

@@ -228,11 +228,11 @@ static int siff_read_packet(AVFormatContext *s, AVPacket *pkt)
 }
 
 AVInputFormat ff_siff_demuxer = {
-    "siff",
-    NULL_IF_CONFIG_SMALL("Beam Software SIFF"),
-    sizeof(SIFFContext),
-    siff_probe,
-    siff_read_header,
-    siff_read_packet,
+    .name           = "siff",
+    .long_name      = NULL_IF_CONFIG_SMALL("Beam Software SIFF"),
+    .priv_data_size = sizeof(SIFFContext),
+    .read_probe     = siff_probe,
+    .read_header    = siff_read_header,
+    .read_packet    = siff_read_packet,
     .extensions = "vb,son"
 };