linux-atm: Fix compile problem with kernel 4.20

linux-atm: Fix compile problem with kernel 4.20

This fixes the following compile problem with kernel 4.20:

In file included from arp.c:20:0:
include/linux/if_arp.h:121:16: error: 'IFNAMSIZ' undeclared here (not in a function)
  char  arp_dev[IFNAMSIZ];
                ^~~~~~~~
make[7]: *** [Makefile:459: arp.o] Error 1

This is caused by commit 6a12709da354 ("net: if_arp: use define instead
of hard-coded value") in the upstream Linux kernel which is integrated
in Linux 4.20.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>