I don't really understand why it depends on evdev xf86 driver. It is not really needed, if I use xf86-input-libinput. equery d xf86-input-evdev * These packages depend on xf86-input-evdev: kde-plasma/plasma-desktop-5.14.5.1 (mouse ? x11-drivers/xf86-input-evdev)
But it is needed. By the plasma-desktop build system.
Just for the record, specifically kcms/mouse/backends/x11/x11_evdev_backend.cpp requires evdev-properties.h - it's a mandatory dependency from upstream.