xchange-93: Use VK_ prefix on DEBUG_REPORT_..._NAME

Use the Vulkan namespace (VK_ prefix) on DEBUG_REPORT_EXTENSION_NAME
This resolves LunarXchange bug #93.
https://vulkan.lunarg.com/app/issues/55b76ae37ef24d0001000070
diff --git a/demos/cube.c b/demos/cube.c
index ba155ae..6b41aff 100644
--- a/demos/cube.c
+++ b/demos/cube.c
@@ -2213,9 +2213,9 @@
             WSIextFound = 1;
             extension_names[enabled_extension_count++] = "VK_WSI_swapchain";
         }
-        if (!strcmp(DEBUG_REPORT_EXTENSION_NAME, instance_extensions[i].extName)) {
+        if (!strcmp(VK_DEBUG_REPORT_EXTENSION_NAME, instance_extensions[i].extName)) {
             if (demo->validate) {
-                extension_names[enabled_extension_count++] = DEBUG_REPORT_EXTENSION_NAME;
+                extension_names[enabled_extension_count++] = VK_DEBUG_REPORT_EXTENSION_NAME;
             }
         }
         assert(enabled_extension_count < 64);