layers: rename VkDbgObjectType

VkDebugReportObjectTypeLUNARG fits the extension naming requirements
diff --git a/layers/mem_tracker.h b/layers/mem_tracker.h
index d32690b..116306d 100644
--- a/layers/mem_tracker.h
+++ b/layers/mem_tracker.h
@@ -99,7 +99,7 @@
 // Simple struct to hold handle and type of object so they can be uniquely identified and looked up in appropriate map
 struct MT_OBJ_HANDLE_TYPE {
     uint64_t        handle;
-    VkDbgObjectType type;
+    VkDebugReportObjectTypeLUNARG type;
 };
 
 // Data struct for tracking memory object