ieee802154: add __init to lowpan_frags_sysctl_register

lowpan_frags_sysctl_register is only called by __init lowpan_net_frag_init
(part of the lowpan module).

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>