make dynlink.lo depend on reloc.h in makefile

this is mainly a development convenience but will also ensure users
building from latest git always get up-to-date arch-specific dynamic
linker code without having to "make clean".
diff --git a/Makefile b/Makefile
index ad701c9..683850d 100644
--- a/Makefile
+++ b/Makefile
@@ -77,6 +77,8 @@
 include/bits/alltypes.h: include/bits/alltypes.h.sh
 	sh $< > $@
 
+src/ldso/dynlink.lo: arch/$(ARCH)/reloc.h
+
 %.o: $(ARCH)/%.s
 	$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $<