Follow the docs at Documentation/bpf/kfuncs.rst:
- declare the function with `__bpf_kfunc`
- disables missing prototype warnings, which allows to remove them from
  include/linux/hid-bpf.h

Removing the prototypes is not an issue because we currently have to
redeclare them when writing the BPF program. They will eventually be
generated by bpftool directly AFAIU.

Link: https://lore.kernel.org/r/20240124-b4-hid-bpf-fixes-v2-3-052520b1e5e6@kernel.orgSigned-off-by: Benjamin Tissoires <bentiss@kernel.org>