layers: Add extension Validation so loader will add layers with Validation

Also fix extension supported list in DrawState and ObjectTracker to include
their local extensions in GetGlobalExtensionInfo()
diff --git a/layers/mem_tracker.cpp b/layers/mem_tracker.cpp
index 152a4e9..fe48633 100644
--- a/layers/mem_tracker.cpp
+++ b/layers/mem_tracker.cpp
@@ -878,10 +878,11 @@
     uint32_t version;
     const char * const name;
 };
-#define MEM_TRACKER_LAYER_EXT_ARRAY_SIZE 1
+#define MEM_TRACKER_LAYER_EXT_ARRAY_SIZE 2
 static const struct extProps mtExts[MEM_TRACKER_LAYER_EXT_ARRAY_SIZE] = {
     // TODO what is the version?
-    0x10, "MemTracker"
+    0x10, "MemTracker",
+    0x10, "Validation"
 };
 
 VK_LAYER_EXPORT VkResult VKAPI vkGetGlobalExtensionInfo(