Makefile.include: do not create BINDIR for usepkg build

BINDIR has already have been created by `pkg-prepare` and the real build
directory for the package will be created by the make target itself.

Makefile.include

@@ -476,7 +476,6 @@ pkg-prepare:
 	-@for i in $(USEPKG) ; do "$(MAKE)" -C $(RIOTPKG)/$$i prepare ; done
 
 $(USEPKG:%=$(BINDIR)/%.a): $(BINDIR)/%.a: $(BUILDDEPS) FORCE
-	@mkdir -p $(BINDIR)
 	$(QQ)"$(MAKE)" -C $(RIOTPKG)/$*
 
 clean: