Cast function item to function pointer in order to appease compiler.

This is necessary because of compiler changes. For further information
look at rust-lang/rust#19925.

src/sched.rs

@@ -205,7 +205,10 @@ pub fn clone(mut cb: CloneCb, stack: &mut [u8], flags: CloneFlags) -> Result<pid
 
     let res = unsafe {
         let ptr = stack.as_mut_ptr().offset(stack.len() as isize);
-        ffi::clone(mem::transmute(callback), ptr as *mut c_void, flags.bits(), &mut cb)
+        ffi::clone(mem::transmute(callback as extern "C" fn(*mut Box<::std::ops::FnMut() -> isize>) -> i32),
+                   ptr as *mut c_void,
+                   flags.bits(),
+                   &mut cb)
     };
 
     Errno::result(res)