fix failures found by randomconfig builds

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/in_ether.c b/libbb/in_ether.c
index dadadba..1de383b 100644
--- a/libbb/in_ether.c
+++ b/libbb/in_ether.c
@@ -3,6 +3,7 @@
  * Utility routines.
  */
 
+//kbuild:lib-$(CONFIG_ARP) += in_ether.o
 //kbuild:lib-$(CONFIG_IFCONFIG) += in_ether.o
 //kbuild:lib-$(CONFIG_IFENSLAVE) += in_ether.o