include: Change debug report pUserData parameter to non-const

This was inconsistently defined.
diff --git a/loader/debug_report.c b/loader/debug_report.c
index 7983a5d..2cb7bc3 100644
--- a/loader/debug_report.c
+++ b/loader/debug_report.c
@@ -131,7 +131,7 @@
         VkInstance                          instance,
         VkFlags                             msgFlags,
         const PFN_vkDbgMsgCallback          pfnMsgCallback,
-        const void*                         pUserData,
+        void*                               pUserData,
         VkDbgMsgCallback*                   pMsgCallback)
 {
     VkDbgMsgCallback *icd_info;