API: libcib: Deprecate T_CIB_DIFF_NOTIFY

This could be used with the add_notify_callback() and
del_notify_callback() methods of cib_api_operations_t. However, it's
never been advertised or documented, all other CIB notify event types
are internal, and in the foreseeable future the CIB public API should go
away in favor of new libpacemaker functions.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>