si-standard-link: allow linking MONO capture clients

In case we have a multi-channel capture device and a mono client,
treat the MONO port as UNKNOWN and let it link with the first
port available on the device

Easy to test with arecord, which by default captures in MONO

modules/module-si-standard-link.c

@@ -309,7 +309,8 @@ create_links (WpSiStandardLink * self, WpTransition * transition,
       if (link_all ||
           out_channel == in_channel ||
           out_channel == SPA_AUDIO_CHANNEL_UNKNOWN ||
-          in_channel == SPA_AUDIO_CHANNEL_UNKNOWN)
+          in_channel == SPA_AUDIO_CHANNEL_UNKNOWN ||
+          in_channel == SPA_AUDIO_CHANNEL_MONO)
       {
         g_autoptr (WpProperties) props = NULL;
         WpLink *link;