Fix build on nightly

Nightly recently fixed some soundness issues related to `Sized`. This had to be fixed in the cocoa crates as well, and also affects our ios bindings.

src/platform/ios/ffi.rs

@@ -66,7 +66,7 @@ extern {
     pub fn longjmp(env: *mut libc::c_void, val: libc::c_int);
 }
 
-pub trait NSString {
+pub trait NSString: Sized {
     unsafe fn alloc(_: Self) -> id {
         msg_send![class("NSString"), alloc]
     }