Fix compiler warning from comparing signed to unsigned

std::vector::size returns size_t, which is unsigned.
Change-Id: I81ba3d58a1bd3a3c3d90c911263a5c0df0441b16
Reviewed-by: Karsten Heimrich <karsten.heimrich@qt.io>

tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp

@@ -1798,7 +1798,7 @@ void tst_QtConcurrentMap::stlContainers()
     vector.push_back(2);
 
     std::vector<int> vector2 = QtConcurrent::blockingMapped(vector, mapper);
-    QCOMPARE(vector2.size(), 2);
+    QCOMPARE(vector2.size(), 2u);
 
     std::list<int> list;
     list.push_back(1);