fix a thinko in build system

diff --git a/networking/udhcp/Kbuild b/networking/udhcp/Kbuild
index 7d47470..f4be6df 100644
--- a/networking/udhcp/Kbuild
+++ b/networking/udhcp/Kbuild
@@ -15,7 +15,7 @@
                                    script.o
 
 UDHCPC_NEEDS_ARPING-$(CONFIG_FEATURE_UDHCPC_ARPING) = y
-lib-$(UDHCPC_NEEDS_ARPING)      += arpping.o
+lib-$(UDHCPC_NEEDS_ARPING-y)    += arpping.o
 
 lib-$(CONFIG_APP_UDHCPD)        += dhcpd.o arpping.o files.o leases.o \
                                    serverpacket.o static_leases.o