Instead add the character to the snprintf above as suggested by Mark.

Silences a warning:
libavformat/matroskadec.c: In function 'webm_dash_manifest_cues':
libavformat/matroskadec.c:3947:13: warning: 'strncat' specified bound 1 equals source length [-Wstringop-overflow=]
             strncat(buf, ",", 1);
             ^~~~~~~~~~~~~~~~~~~~