staging: lustre: remove proc_dir_entry_t typedef

Use struct proc_dir_entry like the rest of the kernel does.

Cc: Peng Tao <tao.peng@emc.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>