png_parser: use designated initializers

Signed-off-by: NPaul B Mahol <onemda@gmail.com>

libavcodec/png_parser.c

@@ -115,9 +115,8 @@ flush:
 }
 
 AVCodecParser ff_png_parser = {
-    { CODEC_ID_PNG },
-    sizeof(PNGParseContext),
-    NULL,
-    png_parse,
-    ff_parse_close,
+    .codec_ids      = { CODEC_ID_PNG },
+    .priv_data_size = sizeof(PNGParseContext),
+    .parser_parse   = png_parse,
+    .parser_close   = ff_parse_close,
 };