sign-file: use 'struct module_signature' from the UAPI headers

sign-file: use 'struct module_signature' from the UAPI headers

Now that the UAPI headers provide the required definitions, use those.
Some symbols have been renamed, adapt to those.

Also adapt the include path for the custom sign-file rule in the
bpf selftests.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>