GPU: Add vulkan to GPU_backend_get_type().

Vulkan backend detection wasn't added to GPU_backend_get_type.
This change will add support for vulkan to the function.

source/blender/gpu/intern/gpu_context.cc

@@ -321,6 +321,12 @@ eGPUBackendType GPU_backend_get_type()
   }
 #endif
 
+#ifdef WITH_VULKAN_BACKEND
+  if (g_backend && dynamic_cast<VKBackend *>(g_backend) != nullptr) {
+    return GPU_BACKEND_VULKAN;
+  }
+#endif
+
   return GPU_BACKEND_NONE;
 }