QSettings: prepare for QChar::unicode() return type change

The return type will change from ushort to char16_t, and while for
the vast majority of the users, this will be a non-breaking change,
this code here would fail.

Fix by using an auto variable, as it should have been from the onset.

Change-Id: I8e3a0f3b38d83434665ee50aa8946b0edbcf133a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qsettings_win.cpp

@@ -102,7 +102,7 @@ static QString escapedKey(QString uKey)
     QChar *data = uKey.data();
     int l = uKey.length();
     for (int i = 0; i < l; ++i) {
-        ushort &ucs = data[i].unicode();
+        auto &ucs = data[i].unicode();
         if (ucs == '\\')
             ucs = '/';
         else if (ucs == '/')