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>