sokol_app.h win32: fix unused warning in release mode (clang)

sokol_app.h

@@ -6686,7 +6686,7 @@ _SOKOL_PRIVATE void _sapp_d3d11_create_default_render_target(void) {
     SOKOL_ASSERT(0 == _sapp.d3d11.ds);
     SOKOL_ASSERT(0 == _sapp.d3d11.dsv);
 
-    HRESULT hr;
+    HRESULT hr; _SOKOL_UNUSED(hr);
 
     /* view for the swapchain-created framebuffer */
     hr = _sapp_dxgi_GetBuffer(_sapp.d3d11.swap_chain, 0, _sapp_win32_refiid(_sapp_IID_ID3D11Texture2D), (void**)&_sapp.d3d11.rt);