net: ipmr: make function ipmr_notifier_init static

The function ipmr_notifier_init is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
warning: symbol 'ipmr_notifier_init' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>