The kernel headers used by the "kobo" koxtoolchain target are *very* old (purposefully, they *need* to match the kernel used by the earliest supported device), so, yeah, it's missing a few <linux/input.h> stuff you'd think would have been there forever
.
You can either ship an up-to-date header (like libevdev does),
patch up the missing defines, or use the kobomk7 koxtoolchain target, which is built against a much less idiotic kernel, a slightly less idiotic glibc, and should still effectively work just fine on 100% of devices, assuming they're running FW >= 4.6. That's a trade we can't make for KOreader, because there's a surprising amount of people on old devices still running FW 3.x, but the target exists specifically to prevent technical debt like that for new stuff.