loader: Rename DevExt to dev_ext

Make a quick change to conform a structure element to the coding
standard.

Change-Id: Icbf168b1ab451dcc4847a0c2c17d0cb40c0197b8
diff --git a/loader/table_ops.h b/loader/table_ops.h
index 2b4d6d4..cb84b87 100644
--- a/loader/table_ops.h
+++ b/loader/table_ops.h
@@ -45,7 +45,7 @@
                                   PFN_vkGetDeviceProcAddr gpa, VkDevice dev) {
     VkLayerDispatchTable *table = &dev_table->core_dispatch;
     for (uint32_t i = 0; i < MAX_NUM_DEV_EXTS; i++)
-        dev_table->ext_dispatch.DevExt[i] = (PFN_vkDevExt)vkDevExtError;
+        dev_table->ext_dispatch.dev_ext[i] = (PFN_vkDevExt)vkDevExtError;
 
     table->GetDeviceProcAddr =
         (PFN_vkGetDeviceProcAddr)gpa(dev, "vkGetDeviceProcAddr");