This commit adds a fix for XPPen Artist 24 where the second button on
the pen is used as an eraser.

It's a "feature" from Microsoft, but it turns out that it's actually
painful for artists. So we ship here a HID-BPF program that turns this
second button into an actual button.

Note that the HID-BPF program is not directly loaded by the kernel itself
but by udev-hid-bpf[0]. But having the sources here allows us to also
integrate tests into tools/testing/selftests/hid to ensure the HID-BPF
program are actually tested.

[0] https://gitlab.freedesktop.org/libevdev/udev-hid-bpf

Link: https://lore.kernel.org/r/20240410-bpf_sources-v1-2-a8bf16033ef8@kernel.orgReviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>