vulkan: constify LayerRef members

Consitfy GetSpecVersion and layer_.

Change-Id: Ieaa4e463ca4666fc9f81b9904fe0783c55e98d03
diff --git a/vulkan/libvulkan/layers_extensions.cpp b/vulkan/libvulkan/layers_extensions.cpp
index 08038b7..93e83af 100644
--- a/vulkan/libvulkan/layers_extensions.cpp
+++ b/vulkan/libvulkan/layers_extensions.cpp
@@ -424,7 +424,7 @@
     return GetLayerRef(g_device_layers, name);
 }
 
-LayerRef::LayerRef(Layer* layer) : layer_(layer) {}
+LayerRef::LayerRef(const Layer* layer) : layer_(layer) {}
 
 LayerRef::~LayerRef() {
     if (layer_) {
@@ -437,7 +437,7 @@
     return layer_->properties.layerName;
 }
 
-uint32_t LayerRef::GetSpecVersion() {
+uint32_t LayerRef::GetSpecVersion() const {
     return layer_->properties.specVersion;
 }