cleanup: remove pppoe_ioctl() declaration.

Hi,
  - This patch removes pppoe_ioctl()  declaration in
drivers/net/pppoe.c as it is unneeded.

Regards,
Rami Rosen

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>