selftests/hid: fix kfunc inclusions with newer bpftool

bpftool now embeds the kfuncs definitions directly in the generated
vmlinux.h

This is great, but because the selftests dir might be compiled with
HID_BPF disabled, we have no guarantees to be able to compile the
sources with the generated kfuncs.

If we have the kfuncs, because we have the `__not_used` hack, the newly
defined kfuncs do not match the ones from vmlinux.h and things go wrong.

Prevent vmlinux.h to define its kfuncs and also add the missing `__weak`
symbols for our custom kfuncs definitions

Link: https://patch.msgid.link/20241128-fix-new-bpftool-v1-1-c9abdf94a719@kernel.org


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>