[DirectShow] Fix potention undefined behavior on destruction

The exact warning is:
> deleting object of polymorphic class type which has non-virtual
  destructor might cause undefined behavior

Change-Id: I8f259def3f1f7f776fb31ace9dfcc7145426f5c0
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>

src/plugins/directshow/player/directshowiosource.h

@@ -49,7 +49,7 @@ class DirectShowIOSource
 {
 public:
     DirectShowIOSource(DirectShowEventLoop *loop);
-    ~DirectShowIOSource();
+    virtual ~DirectShowIOSource();
 
     void setDevice(QIODevice *device);
     void setAllocator(IMemAllocator *allocator);