Remove version.exe with a -f to avoid a warning.

If version.exe does not exist and we don't have a -f then we
will get a warning.

Makefile.in

@@ -107,7 +107,7 @@ clean:
 	rm -f syn-rules.output dosify.exe ivl@EXEEXT@ check.vvp
 	rm -f lexor_keyword.cc libivl.a libvpi.a iverilog-vpi syn-rules.cc
 	rm -rf dep
-	rm version.exe
+	rm -f version.exe
 
 distclean: clean
 	$(foreach dir,$(SUBDIRS),$(MAKE) -C $(dir) $@ && ) true