arptables: needs mmu
arptables uses fork() and breaks on !mmu.
Fixes
http://autobuild.buildroot.net/results/fe18dd4a0b0d17404560abf3e4a332b58751f5cb/
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>