Reimplement QAbstractFileEngine::open() properly

The virtual function takes a QIODevice::OpenMode argument, not an int.

tests/auto/corelib/io/qfile/test/../tst_qfile.cpp:1942: warning:
'MyEngine::open' hides overloaded virtual function [-Woverloaded-
virtual]
    bool open(int ) { return false; }
         ^
src/corelib/io/qabstractfileengine.h:118: hidden overloaded virtual
function 'QAbstractFileEngine::open' declared here
    virtual bool open(QIODevice::OpenMode openMode);
                 ^

Change-Id: I92338dacb89c05d8c5cfbf8ce094dc519b84d3ba
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>

tests/auto/corelib/io/qfile/tst_qfile.cpp

@@ -1939,7 +1939,7 @@ public:
     virtual ~MyEngine() {}
 
     void setFileName(const QString &) {}
-    bool open(int ) { return false; }
+    bool open(QIODevice::OpenMode) { return false; }
     bool close() { return false; }
     bool flush() { return false; }
     qint64 size() const { return 123 + number; }