xfree86: [v2] Remove duplicate InputInfoPtr typedef from xf86Xinput.h

GCC 4.2 doesn't accept 2 typedef declarations of the same type, so
remove the extra one from xf86Xinput.h and have xf86Xinput.h #include
xf86.h to make sure everyone using just that file gets the typedef.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Matthieu Herrb <matthieu@herrb.eu>