Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I7e061f68acdde368a3b63c029b928133646ebff2
diff --git a/vulkan/libvulkan/layers_extensions.cpp b/vulkan/libvulkan/layers_extensions.cpp
index f9b1002..78c8620 100644
--- a/vulkan/libvulkan/layers_extensions.cpp
+++ b/vulkan/libvulkan/layers_extensions.cpp
@@ -61,7 +61,7 @@
 
 class LayerLibrary {
    public:
-    LayerLibrary(const std::string& path)
+    explicit LayerLibrary(const std::string& path)
         : path_(path), dlhandle_(nullptr), refcount_(0) {}
 
     LayerLibrary(LayerLibrary&& other)