layers: Add missing <string> dependency

Change-Id: Ib3c74be1e1d3c168720aaef4dafed1a6a04c7d9b
diff --git a/layers/vk_layer_config.h b/layers/vk_layer_config.h
index f64c547..2f68054 100644
--- a/layers/vk_layer_config.h
+++ b/layers/vk_layer_config.h
@@ -20,6 +20,7 @@
 #pragma once
 #include "vulkan/vulkan.h"
 #include "vulkan/vk_layer.h"
+#include <string>
 #include <unordered_map>
 #include <stdbool.h>
 #include <stdio.h>