Use correct oggz argument type in examples.

The examples using oggz must use oggz_package as the
second argument to read_package for the callback to be acceptable by
oggz_set_read_callback().  Adjust the method signature to use
oggz_package and extract the required ogg_package members from this
one to avoid compiler message like this:

fishsound-identify.c: In function ‘main’:
fishsound-identify.c:101:37: warning: passing argument 3 of ‘oggz_set_read_callback’ from incompatible pointer type [-Wincompatible-pointer-types]
  101 |   oggz_set_read_callback (oggz, -1, read_packet, NULL);
      |                                     ^~~~~~~~~~~
      |                                     |
      |                                     int (*)(OGGZ *, ogg_packet *, long int,  void *) {aka int (*)(void *, ogg_packet *, long int,  void *)}
In file included from /usr/include/oggz/oggz.h:576,
                 from fishsound-identify.c:39:
/usr/include/oggz/oggz_read.h:109:44: note: expected ‘OggzReadPacket’ {aka ‘int (*)(void *, oggz_packet *, long int,  void *)’} but argument is of type ‘int (*)(OGGZ *, ogg_packet *, long int,  void *)’ {aka ‘int (*)(void *, ogg_packet *, long int,  void *)’}
  109 |                             OggzReadPacket read_packet, void * user_data);
      |                             ~~~~~~~~~~~~~~~^~~~~~~~~~~

Fixes #13