QReadWriteLock: make field 'recursive' const

It is never written after initialization, and accessed concurrently
outside mutex protection, so make sure that it never changes by
marking it as const.

Change-Id: Ib28b2e8b3067d596a95d1699165a6ebfb15fe674
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/thread/qreadwritelock_p.h

@@ -74,7 +74,7 @@ public:
     int writerCount;
     int waitingReaders;
     int waitingWriters;
-    bool recursive;
+    const bool recursive;
 
     //Called with the mutex locked
     bool lockForWrite(int timeout);