Move the vk::to_string functions into a separate header vulkan_to_string.hpp.

vulkan/vulkan.hpp

@@ -5600,6 +5600,9 @@ namespace VULKAN_HPP_NAMESPACE
 }  // namespace VULKAN_HPP_NAMESPACE
 
 #include <vulkan/vulkan_enums.hpp>
+#if !defined( VULKAN_HPP_NO_TO_STRING )
+#  include <vulkan/vulkan_to_string.hpp>
+#endif
 
 #ifndef VULKAN_HPP_NO_EXCEPTIONS
 namespace std