ff_id3v2_parse_apic/chapters/priv/priv_dict all had a parameter
extra_meta of type ID3v2ExtraMeta ** as if the functions wanted to make
*extra_meta point to something else. But they don't, so just use an
ID3v2ExtraMeta *.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>