Add magic for Musepack SV8

According to http://trac.musepack.net/musepack/wiki/SV8Specification,
the file starts with a 'MPCK' magic.

Currently, only the SV7 magic, 'MP+' is recognized:
http://trac.musepack.net/musepack/wiki/SV7Specification.

Fixes #104.
1 job for master in 1 minute and 25 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #2581202
build:meson

00:01:25