layers: Make unique_objects ID map global

Increasingly, instance objects are referenced by device-level APIs.
Made unique ID map global, shared across all instances and devices.

Change-Id: I5b62e7b95ec25a2a8b390d46024b43c29f2199ce
diff --git a/layers/unique_objects.h b/layers/unique_objects.h
index 0a03234..cbb4c05 100644
--- a/layers/unique_objects.h
+++ b/layers/unique_objects.h
@@ -46,6 +46,7 @@
 };
 
 struct instance_layer_data {
+
     VkInstance instance;
 
     debug_report_data *report_data;