vulkan: constify DebugReportCallbackList::Message

Sending a message should not modify the state of DebugReportCallbackList,
conceptually.

Bug: 28120066
Change-Id: I7b30b4a41492b4e670408f34a6bffe8b56295afd
diff --git a/vulkan/libvulkan/debug_report.cpp b/vulkan/libvulkan/debug_report.cpp
index 0388fdc..988655e 100644
--- a/vulkan/libvulkan/debug_report.cpp
+++ b/vulkan/libvulkan/debug_report.cpp
@@ -58,9 +58,9 @@
                                       size_t location,
                                       int32_t message_code,
                                       const char* layer_prefix,
-                                      const char* message) {
+                                      const char* message) const {
     std::shared_lock<decltype(rwmutex_)> lock(rwmutex_);
-    Node* node = &head_;
+    const Node* node = &head_;
     while ((node = node->next)) {
         if ((node->flags & flags) != 0) {
             node->callback(flags, object_type, object, location, message_code,