Add alt url to Anders Gavare

This is specific to their winning entry (which is actually referred to
in the index.html file).

author/Anders_Gavare.json

@@ -7,7 +7,7 @@
     "location_code" : "SE",
     "email" : "gavare@gmail.com",
     "url" : "https://gavare.se",
-    "alt_url" : null,
+    "alt_url" : "https://gavare.se/ioccc/ioccc_gavare.c.html",
     "deprecated_twitter_handle" : null,
     "mastodon" : null,
     "mastodon_url" : null,