Fix google-explicit-constructor warnings in vulkan.

* Add explicit keyword to conversion constructors.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I13c32bd5bc53fe897df58eda331ff70ebcf0b349
diff --git a/vulkan/libvulkan/layers_extensions.h b/vulkan/libvulkan/layers_extensions.h
index 79fe59d..07ac1a3 100644
--- a/vulkan/libvulkan/layers_extensions.h
+++ b/vulkan/libvulkan/layers_extensions.h
@@ -26,7 +26,7 @@
 
 class LayerRef {
    public:
-    LayerRef(const Layer* layer);
+    explicit LayerRef(const Layer* layer);
     LayerRef(LayerRef&& other);
     ~LayerRef();
     LayerRef(const LayerRef&) = delete;