Include <string> for fixing undefined template on Mac.

Change-Id: I4e8a860446bde7f76ea289888bf592d78c59b9c4
Reviewed-on: https://swiftshader-review.googlesource.com/21008
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Merck Hung <merckhung@google.com>
diff --git a/src/Vulkan/VkGetProcAddress.cpp b/src/Vulkan/VkGetProcAddress.cpp
index ea5f8c8..b7a51cb 100644
--- a/src/Vulkan/VkGetProcAddress.cpp
+++ b/src/Vulkan/VkGetProcAddress.cpp
@@ -15,6 +15,7 @@
 #include "VkGetProcAddress.h"
 
 #include <unordered_map>
+#include <string>
 
 namespace vk
 {