QIODeviceBase: make dtor protected

Base class dtors should be virtual and public or else protected and
non-virtual. This one was neither of these.

[ChangeLog][Potentially Source-Incompatible Changes][QtCore] The
QIODeviceBase destructor is now protected to avoid deleting objects of
classes derived from it through a QIODeviceBase pointer, which would
be undefined behavior.

Change-Id: Id390ace75d5b64f746d9b1865b8800c9f0590977
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qiodevicebase.h

@@ -46,6 +46,8 @@ QT_BEGIN_NAMESPACE
 
 class QIODeviceBase
 {
+protected:
+    ~QIODeviceBase() = default;
 public:
     enum OpenModeFlag {
         NotOpen = 0x0000,