Adjust dispatch C function

[karlpip]

Since i updated libvlc today, i could not build libvlc-go without warnings anymore.

../libvlc-go/v3/event_manager.go: In function 'eventAttach':
../libvlc-go/v3/event_manager.go:10:40: error: passing argument 3 of 'libvlc_event_attach' from incompatible pointer type [-Wincompatible-pointer-types]
   10 |     return libvlc_event_attach(em, et, eventDispatch, (void*)userData);
      |                                        ^~~~~~~~~~~~~
      |                                        |
      |                                        void (*)(libvlc_event_t *, void *)
In file included from /usr/include/vlc/vlc.h:40,
                 from ../libvlc-go/v3/event_manager.go:5:
/usr/include/vlc/libvlc.h:339:59: note: expected 'libvlc_callback_t' {aka 'void (*)(const struct libvlc_event_t *, void *)'} but argument is of type 'void (*)(libvlc_event_t *, void *)'
  339 |                                         libvlc_callback_t f_callback,
      |                                         ~~~~~~~~~~~~~~~~~~^~~~~~~~~~
../libvlc-go/v3/event_manager.go: In function 'eventDetach':
../libvlc-go/v3/event_manager.go:13:33: error: passing argument 3 of 'libvlc_event_detach' from incompatible pointer type [-Wincompatible-pointer-types]
   13 |     libvlc_event_detach(em, et, eventDispatch, (void*)userData);
      |                                 ^~~~~~~~~~~~~
      |                                 |
      |                                 void (*)(libvlc_event_t *, void *)
/usr/include/vlc/libvlc.h:352:60: note: expected 'libvlc_callback_t' {aka 'void (*)(const struct libvlc_event_t *, void *)'} but argument is of type 'void (*)(libvlc_event_t *, void *)'
  352 |                                          libvlc_callback_t f_callback,
      |                                          ~~~~~~~~~~~~~~~~~~^~~~~~~~~~

This PR adjusts the dispatch functions signature in order to fix the warnings.

See: https://github.com/videolan/vlc/blob/master/include/vlc/libvlc.h#L325

[adrg]

@karlpip Thank you for the PR. Merged. I also added you to the contributors list in README.md.