Silence Clazy's warning in QCborContainerPrivate::insertAt()

Use more suitable QList::insert() overload and silence Clazy's
"mixing iterators" warning. The warning is non-sense for this
particular place, but it won't harm to avoid C-style cast and
iterators arithmetics there.

Change-Id: Ibfc8c8003473de3b7d9b67965e25a4cdb6a9f043
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qcborvalue_p.h

@@ -195,7 +195,7 @@ public:
     }
     void insertAt(qsizetype idx, const QCborValue &value, ContainerDisposition disp = CopyContainer)
     {
-        replaceAt_internal(*elements.insert(elements.begin() + int(idx), {}), value, disp);
+        replaceAt_internal(*elements.insert(idx, {}), value, disp);
     }
 
     void append(QtCbor::Undefined)