layers: Update 'string' layer header files

Remove struct_string_helper header files, add in enum_string_helper
and <sstream> as these were included in the removed header file.

Change-Id: Ice0842ce2f06b5581a5add739534d59d834edbf6
diff --git a/layers/core_validation.cpp b/layers/core_validation.cpp
index 44ad72b..9842ace 100644
--- a/layers/core_validation.cpp
+++ b/layers/core_validation.cpp
@@ -35,7 +35,7 @@
 #include <map>
 #include <mutex>
 #include <set>
-//#include <memory>
+#include <sstream>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -44,7 +44,7 @@
 
 #include "vk_loader_platform.h"
 #include "vk_dispatch_table_helper.h"
-#include "vk_struct_string_helper_cpp.h"
+#include "vk_enum_string_helper.h"
 #if defined(__GNUC__)
 #pragma GCC diagnostic ignored "-Wwrite-strings"
 #endif
diff --git a/layers/image.cpp b/layers/image.cpp
index 373ae37..5899dcf 100644
--- a/layers/image.cpp
+++ b/layers/image.cpp
@@ -36,10 +36,11 @@
 #include <unordered_map>
 #include <vector>
 #include <bitset>
+#include <sstream>
 
 #include "vk_loader_platform.h"
 #include "vk_dispatch_table_helper.h"
-#include "vk_struct_string_helper_cpp.h"
+#include "vk_enum_string_helper.h"
 #include "image.h"
 #include "vk_layer_config.h"
 #include "vk_layer_extension_utils.h"
diff --git a/layers/threading.cpp b/layers/threading.cpp
index 565a8b2..f73774c 100644
--- a/layers/threading.cpp
+++ b/layers/threading.cpp
@@ -31,7 +31,7 @@
 #include "vk_layer_logging.h"
 #include "threading.h"
 #include "vk_dispatch_table_helper.h"
-#include "vk_struct_string_helper_cpp.h"
+#include "vk_enum_string_helper.h"
 #include "vk_layer_data.h"
 #include "vk_layer_utils.h"
 
diff --git a/layers/unique_objects.cpp b/layers/unique_objects.cpp
index bd9aabd..779693e 100644
--- a/layers/unique_objects.cpp
+++ b/layers/unique_objects.cpp
@@ -40,7 +40,7 @@
 #include "vk_layer_table.h"
 #include "vk_layer_utils.h"
 #include "vk_layer_utils.h"
-#include "vk_struct_string_helper_cpp.h"
+#include "vk_enum_string_helper.h"
 #include "vk_validation_error_messages.h"
 #include "vulkan/vk_layer.h"