SDL_vulkan.h: undefine VK_DEFINE_HANDLER and VK_DEFINE_NON_DISPATCHABLE_HANDLE

Allows including vulkan.hpp in user code in C++ before SDL_vulkan.h w/o errors.
Fixes:  https://github.com/libsdl-org/SDL/issues/11328 .

include/SDL3/SDL_vulkan.h

@@ -69,6 +69,10 @@ VK_DEFINE_HANDLE(VkPhysicalDevice)
 VK_DEFINE_NON_DISPATCHABLE_HANDLE(VkSurfaceKHR)
 struct VkAllocationCallbacks;
 
+/* Make sure to undef to avoid issues in case of later vulkan include */
+#undef VK_DEFINE_HANDLE
+#undef VK_DEFINE_NON_DISPATCHABLE_HANDLE
+
 #endif /* !NO_SDL_VULKAN_TYPEDEFS */
 
 /**