selftests/hid: force using our compiled libbpf headers

Turns out that we were relying on the globally installed headers, not
the ones we freshly compiled.
Add a manual include in CFLAGS to sort this out.

Tested-by: Nick Desaulniers <ndesaulniers@google.com> # Build
Tested-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230825-wip-selftests-v3-3-639963c54109@kernel.org


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