Remove two more files in gdb "distclean"

The recent work to have gdb link via libtool means that there are a
couple more generated files in the build directory that should be
removed by "distclean".

Note that gdb can't really fully implement distclean due to the desire
to put certain generated files into the distribution.  Still, it can
get pretty close.

gdb/Makefile.in

@@ -2224,6 +2224,7 @@ distclean: clean
 	rm -f gdb-gdb.py gdb-gdb.gdb
 	rm -f y.output yacc.acts yacc.tmp y.tab.h
 	rm -f config.log config.cache
+	rm -f config.lt libtool
 	rm -f Makefile
 	rm -rf $(DEPDIR)
 	for i in $(CONFIG_SRC_SUBDIR); do \