Change the type of DISPATCH_QUEUE_CONCURRENT.

Turns out I could only get away with this because of a type checking
bug; if you try to add a cast to dispatch_queue_attr_t the compiler will
start to complain about 'shared static variables must have a type that
implements Sync'. Discovered in rust-lang/rust#33130.

src/ffi.rs

@@ -132,7 +132,7 @@ pub fn dispatch_get_main_queue() -> dispatch_queue_t {
 }
 
 pub const DISPATCH_QUEUE_SERIAL: dispatch_queue_attr_t = 0 as dispatch_queue_attr_t;
-pub static DISPATCH_QUEUE_CONCURRENT: dispatch_queue_attr_t = &_dispatch_queue_attr_concurrent;
+pub static DISPATCH_QUEUE_CONCURRENT: &'static dispatch_object_s = &_dispatch_queue_attr_concurrent;
 
 pub const DISPATCH_QUEUE_PRIORITY_HIGH: c_long       = 2;
 pub const DISPATCH_QUEUE_PRIORITY_DEFAULT: c_long    = 0;