rules.mk: make sure we always create a .depend file

There are some cases where "make depend" would always run when
entering a directory.  This happened when both the $(SRCS) and
$(HOSTSRCS) lists were empty (which is for example typical for the
examples/api/ directory).  Avoid this by making sure that a ".depend"
file gets always created, even if empty.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>

rules.mk

@@ -27,6 +27,7 @@ _depend:	$(obj).depend
 
 $(obj).depend:	$(src)Makefile $(TOPDIR)/config.mk $(SRCS) $(HOSTSRCS)
 		@rm -f $@
+		@touch $@
 		@for f in $(SRCS); do \
 			g=`basename $$f | sed -e 's/\(.*\)\.\w/\1.o/'`; \
 			$(CC) -M $(CPPFLAGS) -MQ $(obj)$$g $$f >> $@ ; \