Condition out things specific to some versions of the input extension

Condition out things specific to some versions of the input extension

* src/xterm.c (x_free_xi_devices, x_init_master_valuators)
(handle_one_xevent): Condition out code that isn't supposed to
be run on the current input extension version.
* src/xterm.h (struct xi_scroll_valuator_t): Make conditional on
HAVE_XINPUT2_1.
(struct xi_touch_point_t): Make conditional on HAVE_XINPUT2_2.
(struct xi_device_t): Make individual fields conditional on the
appropriate client-side input extension version.