Fix small mistake in makefile

packet-exchange/build/Makefile

@@ -40,10 +40,10 @@ xdp_kern.o:
 	llc -march=bpf -filetype=obj -o xdp_kern.o xdp_kern.ll
 
 ifneq ($(WITH_XDP),)
-$(SERVER_PROG): $(SERVER_OBJS)
+$(SERVER_PROG): $(SERVER_OBJS) xdp_kern.o
 	$(CC) $(LDFLAGS) $(LDIRS) $^ $(LLIBS) -o $@
 else
-$(SERVER_PROG): $(SERVER_OBJS), xdp_kern.o
+$(SERVER_PROG): $(SERVER_OBJS)
 	$(CC) $(LDFLAGS) $(LDIRS) $^ $(LLIBS) -o $@
 endif