Fix incorrect prototype for ipxrtr_route_packet()

The function ipxrtr_route_packet() takes a 'len' argument of type
size_t. However, its prototype in af_ipx.c incorrectly suggests that the
corresponding argument is of type 'int' instead.

Discovered by building with --combine and letting the compiler see it
all at once.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>