vulkan.h: Added #define for MAX_DESCRIPTION

Part of header revision 135 changes.
diff --git a/include/vulkan.h b/include/vulkan.h
index 6d79fcc..5444b1f 100644
--- a/include/vulkan.h
+++ b/include/vulkan.h
@@ -101,6 +101,7 @@
 
 #define VK_MAX_PHYSICAL_DEVICE_NAME 256
 #define VK_MAX_EXTENSION_NAME       256
+#define VK_MAX_DESCRIPTION          256
 #define VK_MAX_MEMORY_TYPES         32
 #define VK_MAX_MEMORY_HEAPS         16
 #define VK_LOD_CLAMP_NONE           MAX_FLOAT