layers: Fix Memtracker fence-related memory leak

MemTracker created internal fence objects to locally track command buffer
completion, but these objects were never destroyed.
diff --git a/layers/mem_tracker.h b/layers/mem_tracker.h
index 5e26147..930d972 100644
--- a/layers/mem_tracker.h
+++ b/layers/mem_tracker.h
@@ -149,5 +149,6 @@
     uint32_t                        colorAttachmentCount;
     XGL_DEPTH_STENCIL_BIND_INFO     dsBindInfo;
     XGL_CMD_BUFFER cmdBuffer;
-    XGL_FENCE fence; // fence tracking this cmd buffer
+    XGL_FENCE fence;                // fence tracking this cmd buffer
+    bool32_t localFlag;             // fence is internal to layer
 } GLOBAL_CB_NODE;