BUILD: makefile: mark poll and tcploop targets as phony

Since these ones come with their own makefiles, the top-level makefile
cannot decide when they have to be rebuilt, it should always defer the
decision to the compoent's makefile, so we must mark them as phony.
Because of these, they were not updated after a change without calling
a "clean" first.

Makefile

@@ -1043,7 +1043,7 @@ dev/udp/udp-perturb: dev/udp/udp-perturb.o
 	$(cmd_LD) $(LDFLAGS) -o $@ $^ $(LDOPTS)
 
 # rebuild it every time
-.PHONY: src/version.c
+.PHONY: src/version.c dev/poll/poll dev/tcploop/tcploop
 
 src/calltrace.o: src/calltrace.c $(DEP)
 	$(cmd_CC) $(TRACE_COPTS) -c -o $@ $<