Add missing #include <string>

Fixes build failures seen here:
https://travis-ci.org/paulthomson/build-swiftshader/jobs/565093397

Change-Id: Ie03b1dbbbbbcc586dc4dfc7eba85e0e225012a16
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34456
Reviewed-by: Paul Thomson <paulthomson@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
diff --git a/src/Vulkan/VkPipelineCache.hpp b/src/Vulkan/VkPipelineCache.hpp
index 5195dbe..533b56f 100644
--- a/src/Vulkan/VkPipelineCache.hpp
+++ b/src/Vulkan/VkPipelineCache.hpp
@@ -16,11 +16,13 @@
 #define VK_PIPELINE_CACHE_HPP_
 
 #include "VkObject.hpp"
+
 #include <cstring>
 #include <functional>
-#include <memory>
 #include <map>
+#include <memory>
 #include <mutex>
+#include <string>
 #include <vector>
 
 namespace sw