vulkan: constify LayerRef members

Consitfy GetSpecVersion and layer_.

Change-Id: Ieaa4e463ca4666fc9f81b9904fe0783c55e98d03
diff --git a/vulkan/libvulkan/layers_extensions.h b/vulkan/libvulkan/layers_extensions.h
index 7e7bfd3..64fe5ef 100644
--- a/vulkan/libvulkan/layers_extensions.h
+++ b/vulkan/libvulkan/layers_extensions.h
@@ -25,14 +25,14 @@
 struct Layer;
 class LayerRef {
    public:
-    LayerRef(Layer* layer);
+    LayerRef(const Layer* layer);
     LayerRef(LayerRef&& other);
     ~LayerRef();
     LayerRef(const LayerRef&) = delete;
     LayerRef& operator=(const LayerRef&) = delete;
 
     const char* GetName() const;
-    uint32_t GetSpecVersion();
+    uint32_t GetSpecVersion() const;
 
     // provides bool-like behavior
     operator const Layer*() const { return layer_; }
@@ -43,7 +43,7 @@
     bool SupportsExtension(const char* name) const;
 
    private:
-    Layer* layer_;
+    const Layer* layer_;
 };
 
 void DiscoverLayers();