Add design-2.9 to docinput

While design-2.9.rst was already created earlier, it
wasn't registered in the Makefile. Fix this.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

Makefile.am

@@ -436,6 +436,7 @@ docinput = \
 	doc/design-2.6.rst \
 	doc/design-2.7.rst \
 	doc/design-2.8.rst \
+	doc/design-2.9.rst \
 	doc/design-autorepair.rst \
 	doc/design-bulk-create.rst \
 	doc/design-chained-jobs.rst \