loader: Fix gcc 8.1 cast warning

Change-Id: Id8f0301c4f208d185abe2cc3df71021cb4aa29b1
diff --git a/loader/loader.h b/loader/loader.h
index 9ffa3bb..4114dbe 100644
--- a/loader/loader.h
+++ b/loader/loader.h
@@ -165,7 +165,7 @@
     struct loader_dispatch_hash_list list;  // to handle hashing collisions
 };
 
-typedef void(VKAPI_PTR *PFN_vkDevExt)(VkDevice device);
+typedef VkResult(VKAPI_PTR *PFN_vkDevExt)(VkDevice device);
 struct loader_dev_ext_dispatch_table {
     PFN_vkDevExt dev_ext[MAX_NUM_UNKNOWN_EXTS];
 };