Switch BRASERO_TRACK_STREAM_ISRC_TAG from int to string

The International Standard Recording Code (ISRC) is a  12
byte string that is created from two uppercase characters
for the country code, followed by three uppercase charac‐
ters  for  the owner, followed by two digits for the year
of recording followed by five digits for  the  recording
serial number.

https://bugzilla.gnome.org/show_bug.cgi?id=711162