fdo#70371: create findunusedheaders target

- you can now do "make unusedheaders" to look for headers that seem to be used
- also renamed the script in ./bin to follow the conventions there

Change-Id: Ia9487af02effd05bfbc5c3f5c04b928a407b0fa4

Makefile.in

@@ -376,6 +376,9 @@ findunusedcode:
               | grep -v ^WSObject \
               > unusedcode.easy
 
+findunusedheaders:
+	$(SRCDIR)/bin/find-unusedheaders.pl
+
 # the actual running of subsequentcheck is now done in "build" target => faster
 check: build