Core: `QRandomGenerator` - silence msvc warning

silence msvc warning C4244 (possible loss of data)

Change-Id: I1f0be8abc5337e05d3fe6d25b9afd9991f498e01
Pick-to: 6.1 6.2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qrandom.h

@@ -83,7 +83,7 @@ public:
 
     quint32 generate()
     {
-        return _fillRange(nullptr, 1);
+        return quint32(_fillRange(nullptr, 1));
     }
 
     quint64 generate64()