Fix Strict Weak Ordering in qdoc's comparator

If both n1 and n2 are "Private", the comment says they're equivalent,
but the function is actually stating that n1 < n2 AND n2 < n1.

Change-Id: I8e30b32c1e5240551ab1808baf6fc476841dfde4
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

src/tools/qdoc/qdocindexfiles.cpp

@@ -1135,7 +1135,7 @@ bool compareNodes(const Node* n1, const Node* n2)
     // Private nodes can occur in any order since they won't normally be
     // written to the index.
     if (n1->access() == Node::Private && n2->access() == Node::Private)
-        return true;
+        return false;
 
     if (n1->location().filePath() < n2->location().filePath())
         return true;