matroskadec: Mute "Unknown entry" log messages for CueRelativePosition entries

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -475,6 +475,7 @@ static EbmlSyntax matroska_chapters[] = {
 static EbmlSyntax matroska_index_pos[] = {
     { MATROSKA_ID_CUETRACK,           EBML_UINT, 0, offsetof(MatroskaIndexPos,track) },
     { MATROSKA_ID_CUECLUSTERPOSITION, EBML_UINT, 0, offsetof(MatroskaIndexPos,pos)   },
+    { MATROSKA_ID_CUERELATIVEPOSITION,EBML_NONE },
     { MATROSKA_ID_CUEBLOCKNUMBER,     EBML_NONE },
     { 0 }
 };