makedep: For generated headers, parse the source idl instead.

This avoids having to generate all the headers before make depend.
diff --git a/Make.rules.in b/Make.rules.in
index eecbffd..2cba40f 100644
--- a/Make.rules.in
+++ b/Make.rules.in
@@ -226,7 +226,7 @@
 $(SUBDIRS:%=%/__depend__): dummy
 	@cd `dirname $@` && $(MAKE) depend
 
-depend: $(IDL_SRCS:.idl=.h) $(SUBDIRS:%=%/__depend__) dummy
+depend: $(SUBDIRS:%=%/__depend__) dummy
 	$(MAKEDEP) -C$(SRCDIR) -S$(TOPSRCDIR) -T$(TOPOBJDIR) $(EXTRAINCL) $(DEPEND_SRCS)
 
 .PHONY: depend $(SUBDIRS:%=%/__depend__)