QOrderedMutexLocker: fix UB (pointer comparison with <) in static relock()

Comparing pointers that do not point into the same array using
operator< is UB. You need to use std::less<>.

The QOrderedMutexLocker ctor already used std::less to compare
pointers, but the static relock() function was not fixed.

Amends 5007352164.

Change-Id: I584d382391dd5a2af75020a4e77f3e42ee5d5708
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/thread/qorderedmutexlocker_p.h

@@ -93,7 +93,7 @@ public:
         // mtx1 is already locked, mtx2 not... do we need to unlock and relock?
         if (mtx1 == mtx2)
             return false;
-        if (mtx1 < mtx2) {
+        if (std::less<QMutex *>()(mtx1, mtx2)) {
             mtx2->lock();
             return true;
         }