extmod/extmod.mk: Include mdns app source in lwIP build.

extmod/extmod.mk

@@ -117,6 +117,7 @@ CFLAGS_MOD += -DMICROPY_PY_LWIP=1
 $(BUILD)/$(LWIP_DIR)/core/ipv4/dhcp.o: CFLAGS_MOD += -Wno-address
 SRC_MOD += extmod/modlwip.c lib/netutils/netutils.c
 SRC_MOD += $(addprefix $(LWIP_DIR)/,\
+	apps/mdns/mdns.c \
 	core/def.c \
 	core/dns.c \
 	core/inet_chksum.c \