[gz-gui8] Fails to build MinimalScene if gz-rendering8 is built without vulkan support

[oysstu]

The vulkan external device header is guarded by GZ_RENDERING_HAVE_VULKAN
https://github.com/gazebosim/gz-rendering/blob/22b2bbfbe06ac68558d77948d339fbc294876c85/include/gz/rendering/RenderEngineVulkanExternalDeviceStructs.hh#L27-L63

This is essentially only set for builds using OGRE2/ogre-next and vulkan support.
https://github.com/gazebosim/gz-rendering/blob/22b2bbfbe06ac68558d77948d339fbc294876c85/CMakeLists.txt#L134-L159

The build therefore fails for gz-rendering8 with only ogre 1.9 or non-vulkan ogre-next.

Edit: due to the following include

gz-gui/src/plugins/minimal_scene/MinimalScene.cc

Line 43 in 872222a

#include <gz/rendering/RenderEngineVulkanExternalDeviceStructs.hh>

[jennuine]

@iche033 do you mind to take a look?

[jennuine]

Fixed with #589, thanks @iche033 !