layers: rename VkDbgObjectType

VkDebugReportObjectTypeLUNARG fits the extension naming requirements
diff --git a/demos/cube.c b/demos/cube.c
index 8323707..4ebfdbc 100644
--- a/demos/cube.c
+++ b/demos/cube.c
@@ -257,7 +257,7 @@
 
 VkBool32 dbgFunc(
     VkFlags                             msgFlags,
-    VkDbgObjectType                     objType,
+    VkDebugReportObjectTypeLUNARG                     objType,
     uint64_t                            srcObject,
     size_t                              location,
     int32_t                             msgCode,