Makefile.include: make pkgs Makefile.include FORCE instead of PHONY

The rule to rebuild the Makefile.include(s) should be removed at some
point.

Makefile.include

@@ -323,7 +323,7 @@ include $(RIOTBASE)/drivers/Makefile.include
 $(RIOTPKG)/%/Makefile.include::
 	$(Q)"$(MAKE)" -C $(RIOTPKG)/$* Makefile.include
 
-.PHONY: $(USEPKG:%=$(RIOTPKG)/%/Makefile.include)
+$(USEPKG:%=$(RIOTPKG)/%/Makefile.include): FORCE
 -include $(USEPKG:%=$(RIOTPKG)/%/Makefile.include)
 
 # Deduplicate includes without sorting them