tools: ynl: improve the direct-include header guard logic

Przemek suggests that I shouldn't accuse GCC of witchcraft,
there is a simpler explanation for why we need manual define.

scripts/headers_install.sh modifies the guard, removing _UAPI.
That's why including a kernel header from the tree and from
/usr leads to duplicate definitions.

This also solves the mystery of why I needed to include
the header conditionally. I had the wrong guards for most
cases but ethtool.

Suggested-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://lore.kernel.org/r/20230621231719.2728928-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>