debug_report: MR141, Change pUserData to match type used elsewhere

Other uses of pUserData in Vulkan do not have a const qualifier,
remove it for this pUserData.
diff --git a/include/vulkan/vk_layer.h b/include/vulkan/vk_layer.h
index 546e91d..aeb6ab9 100644
--- a/include/vulkan/vk_layer.h
+++ b/include/vulkan/vk_layer.h
@@ -208,7 +208,7 @@
     VkDebugReportCallbackEXT msgCallback;
     PFN_vkDebugReportCallbackEXT pfnMsgCallback;
     VkFlags msgFlags;
-    const void *pUserData;
+    void *pUserData;
     struct VkLayerDbgFunctionNode_ *pNext;
 } VkLayerDbgFunctionNode;