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

It's more readable and less prone to breakage.
diff --git a/libavformat/c93.c b/libavformat/c93.c
index 097565a..6cf448f 100644
--- a/libavformat/c93.c
+++ b/libavformat/c93.c
@@ -193,10 +193,10 @@
 }
 
 AVInputFormat ff_c93_demuxer = {
-    "c93",
-    NULL_IF_CONFIG_SMALL("Interplay C93"),
-    sizeof(C93DemuxContext),
-    probe,
-    read_header,
-    read_packet,
+    .name           = "c93",
+    .long_name      = NULL_IF_CONFIG_SMALL("Interplay C93"),
+    .priv_data_size = sizeof(C93DemuxContext),
+    .read_probe     = probe,
+    .read_header    = read_header,
+    .read_packet    = read_packet,
 };