qt5base: use libinput by default

qt5base: use libinput by default

Due to the switch from hotplug2 to procd, libudev - which libinput
requires - was rendered unusable on OpenWrt/LEDE.
However since there's libudev-fbsd now which is used as a
drop-in-replacement for libudev, we can now make use of libinput again
and therewith avoid the need of passing platform specific env vars
telling the paths to mouse/touchscreen/keyboard (evdev).