include: Change debug report pUserData parameter to non-const

This was inconsistently defined.
diff --git a/loader/debug_report.h b/loader/debug_report.h
index 0c4371d..52cd170 100644
--- a/loader/debug_report.h
+++ b/loader/debug_report.h
@@ -105,7 +105,7 @@
     VkInstance                          instance,
     VkFlags                             msgFlags,
     const PFN_vkDbgMsgCallback          pfnMsgCallback,
-    const void*                         pUserData,
+    void*                               pUserData,
     VkDbgMsgCallback*                   pMsgCallback);
 
 VkResult VKAPI loader_DbgDestroyMsgCallback(