Bug 1822576 - Don't allow shrinking autocomplete-richlistitems. r=Gijs

Since we rely on the richlistbox having a fixed height with multiple
items, then measure (so shrinking breaks that measurement).

Differential Revision: https://phabricator.services.mozilla.com/D172706

toolkit/content/autocomplete.css

@@ -13,6 +13,7 @@
 }
 
 .autocomplete-richlistbox > richlistitem {
+  flex-shrink: 0;
   flex-direction: row;
   overflow: hidden;
 }