config-parser: treat impossible policy type as IGNORED

This silences -Wswitch-default.

Based on part of a patch from Thomas Zimmermann.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=98191

bus/config-parser.c

@@ -1741,6 +1741,7 @@ append_rule_from_element (BusConfigParser   *parser,
       switch (pe->d.policy.type)
         {
         case POLICY_IGNORED:
+        default:
           /* drop the rule on the floor */
           break;