Fix typo in QMutex's free-list's Sizes[3]

A QFreeList's MaxIndex is clearly meant to be the sum of its Sizes;
but the parentheses in MaxIndex - (16 - 128 - 1024) doesn't give the
same as without the parentheses.  Noticed while reviewing a copy of
this code now used by QReadWriteLock (which has this fix).

Change-Id: I9f68171475b3c6edc18896056b5532183fee92b2
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/thread/qmutex.cpp

@@ -642,7 +642,7 @@ const int FreeListConstants::Sizes[FreeListConstants::BlockCount] = {
     16,
     128,
     1024,
-    FreeListConstants::MaxIndex - (16-128-1024)
+    FreeListConstants::MaxIndex - (16 + 128 + 1024)
 };
 
 typedef QFreeList<QMutexPrivate, FreeListConstants> FreeList;