fix sparse warning about non-static function

drivers/net/bonding/bond_main.c:3737:5: warning: symbol
'bond_3ad_xor_xmit' was not declared. Should it be static?
Reviewed-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: Jonathan Toppins <jtoppins@cumulusnetworks.com>
Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>