can: fix sparse warning in af_can.c

Put can_rx_alldev_list into the af_can header to fix the following
sparse warning: net/can/af_can.c:80:22: warning: symbol
'can_rx_alldev_list' was not declared. Should it be static?

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>