test,mmsmspdu: fix warnings with -Wswitch-enum

mmsmspdu.c: In function ‘show_part_info’:
  mmsmspdu.c:86:5: error: enumeration value ‘MM_SMS_ENCODING_UNKNOWN’ not handled in switch [-Werror=switch-enum]
     86 |     switch (encoding) {
        |     ^~~~~~

test/mmsmspdu.c

@@ -93,6 +93,7 @@ show_part_info (MMSmsPart *part)
     case MM_SMS_ENCODING_8BIT:
         g_print ("encoding: 8BIT\n");
         break;
+    case MM_SMS_ENCODING_UNKNOWN:
     default:
         g_print ("encoding: unknown (0x%x)\n", encoding);
         break;