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>

package/arptables/Config.in

@@ -1,5 +1,7 @@
 config BR2_PACKAGE_ARPTABLES
 	bool "arptables"
+	# needs fork()
+	depends on BR2_USE_MMU
 	help
 	  Tool to set up, maintain, and inspect the tables of ARP rules.