extract_appstream: debug if an exported appstream is skipped

We had missing icons for a few apps due to this - if this ever happens
it means something very weird is going on with the export process in
a flatpak, so it's probably worth alerting the repo admin.

Closes: #1367
Approved by: pwithnall

common/flatpak-utils.c

@@ -3869,6 +3869,8 @@ extract_appstream (OstreeRepo   *repo,
 
           if (!g_str_has_prefix (component_id_text, id))
             {
+              g_print ("Skipping mismatched appstream component %s for app %s\n",
+                       component_id_text, id);
               component = component->next_sibling;
               continue;
             }