Fix harmless signed/unsigned comparison warning in Direct2D code

Cast D2DERR_RECREATE_TARGET, which is not of HRESULT type, to it to avoid gcc
warnings.

src/msw/graphicsd2d.cpp

@@ -3345,7 +3345,7 @@ void wxD2DContext::Flush()
 {
     HRESULT result = m_renderTargetHolder->Flush();
 
-    if (result == D2DERR_RECREATE_TARGET)
+    if (result == (HRESULT)D2DERR_RECREATE_TARGET)
     {
         ReleaseDeviceDependentResources();
     }