debug_report: Make pUserData always a const

This prevents compiler warnings.
diff --git a/layers/vk_layer_logging.h b/layers/vk_layer_logging.h
index 5f9fe51..8a38d42 100644
--- a/layers/vk_layer_logging.h
+++ b/layers/vk_layer_logging.h
@@ -300,7 +300,7 @@
     int32_t                             msgCode,
     const char*                         pLayerPrefix,
     const char*                         pMsg,
-    void*                               pUserData)
+    const void*                         pUserData)
 {
     char msg_flags[30];
 
@@ -321,7 +321,7 @@
     int32_t                             msgCode,
     const char*                         pLayerPrefix,
     const char*                         pMsg,
-    void*                               pUserData)
+    const void*                         pUserData)
 {
 #ifdef WIN32
     char msg_flags[30];