selftests/hid: do not manually call headers_install

"make headers" is a requirement before calling make on the selftests
dir, so we should not have to manually install those headers

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com> # Build
Tested-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230825-wip-selftests-v3-2-639963c54109@kernel.org


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