Defined _XSERVER64 in header

See [0], the _XSERVER64 affects the struct HIME_client_handle_S, so it
shall be defined alone with HIME_client_handle_S to prevent struct
conflict.

[0] https://bugs.debian.org/750375
Merge pull request #496 from czchen/bugfix/xserver64

Defined _XSERVER64 in header