fix resource leak in os.(*File).Close

Signed-off-by: Achille Roussel <achille.roussel@gmail.com>

src/syscall/syscall_libc_darwin_amd64.go

@@ -17,6 +17,11 @@ import (
 //export fdopendir$INODE64
 func libc_fdopendir(fd int32) unsafe.Pointer
 
+// int closedir(struct DIR * buf);
+//
+//export closedir
+func libc_closedir(unsafe.Pointer) int32
+
 // int readdir_r(struct DIR * buf, struct dirent *entry, struct dirent **result);
 //
 //export readdir_r$INODE64