l2tp: static functions should not be exported

Causes these build failures on PowerPC:

net/l2tp/l2tp_core.c:1228: error: __ksymtab_l2tp_tunnel_closeall causes a section type conflict
net/l2tp/l2tp_core.c:1228: error: __ksymtab_l2tp_tunnel_closeall causes a section type conflict
net/l2tp/l2tp_core.c:1006: error: __ksymtab_l2tp_xmit_core causes a section type conflict
net/l2tp/l2tp_core.c:1006: error: __ksymtab_l2tp_xmit_core causes a section type conflict
net/l2tp/l2tp_core.c:847: error: __ksymtab_l2tp_udp_recv_core causes a section type conflict
net/l2tp/l2tp_core.c:847: error: __ksymtab_l2tp_udp_recv_core causes a section type conflict

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>