Fix a compiler warning

xf86libinput.c:2457:89: warning: passing argument 1 of
‘libinput_event_pointer_get_axis_source’ from incompatible
pointer type [-Wincompatible-pointer-types]

No function changes due to the binary layout of libinput events but
let's not rely on that.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
2 jobs for wip/compiler-warning in 1 minute and 7 seconds