tools: ynl-gen: allow uapi headers in sub-dirs

Binder places its headers under include/uapi/linux/android/
Make sure replace / with _ in the uAPI header guard, the c_upper()
is more strict and only converts - to _. This is likely a good
constraint to have, to enforce sane naming in enums etc.
But paths may include /.

Signed-off-by: Li Li <dualli@google.com>
Link: https://patch.msgid.link/20241113193239.2113577-2-dualli@chromium.org


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