[clang] avoid error on ObjC model

Summary:
Infer was complaining about a parameter not null checked, which was failing the
model compilation since no errors are allowed on models.

Reviewed By: ezgicicek

Differential Revision: D15453573

fbshipit-source-id: 3bd0df715

infer/models/objc/src/Dispatch_once.m

@@ -14,6 +14,6 @@ typedef void (^dispatch_block_t)(void);
 
 typedef long dispatch_once_t;
 
-static void _dispatch_once(dispatch_once_t* predicate, dispatch_block_t block) {
+static void _dispatch_once(dispatch_once_t* predicate, __nonnull dispatch_block_t block) {
   block();
 }