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/loader/debug_report.c b/loader/debug_report.c
index 7d1a9b3..f949729 100644
--- a/loader/debug_report.c
+++ b/loader/debug_report.c
@@ -128,7 +128,7 @@
                                   msgCode,
                                   pLayerPrefix,
                                   pMsg,
-                                  (void *) pTrav->pUserData)) {
+                                  pTrav->pUserData)) {
                 bail = true;
             }
         }