cache: assert in nl_cache_mngt_register() for valid oo_keygen() function

oo_keygen() requires oo_compare(). Assert in nl_cache_mngt_register().

http://lists.infradead.org/pipermail/libnl/2014-November/001759.html

Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>