Remove mandatory generated files too

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5951)

Configurations/unix-Makefile.tmpl

@@ -408,7 +408,7 @@ libclean:
 
 clean: libclean
 	$(RM) $(PROGRAMS) $(TESTPROGS) $(ENGINES) $(SCRIPTS)
-	$(RM) $(GENERATED)
+	$(RM) $(GENERATED_MANDATORY) $(GENERATED)
 	-$(RM) `find . -name '*{- $depext -}' -a \! -path "./.git/*"`
 	-$(RM) `find . -name '*{- $objext -}' -a \! -path "./.git/*"`
 	$(RM) core