[cost] Fix test: field of object

Summary: Without curly braces, it is declaration of globals, not fields.

Reviewed By: ezgicicek

Differential Revision: D23866604

fbshipit-source-id: dd685c8d6

infer/tests/codetoanalyze/objc/autoreleasepool/arc_enumerator.m

@@ -12,9 +12,10 @@ typedef BOOL (^Filter)(id);
 @interface MyEnumerator : NSObject
 @end
 
-@implementation MyEnumerator
-NSEnumerator* _enumerator;
-Filter _filter;
+@implementation MyEnumerator {
+  NSEnumerator* _enumerator;
+  Filter _filter;
+}
 
 - (instancetype)initWithArray:(NSArray*)array filter:(Filter)filter {
   _enumerator = [array objectEnumerator];