module: Move 'struct module_signature' to UAPI

module: Move 'struct module_signature' to UAPI

This structure definition is used outside the kernel proper.
For example in kmod and the kernel build environment.

To allow reuse, move it to a new UAPI header.

While it is not a true UAPI, it is a common practice to have
non-UAPI interface definitions in the kernel's UAPI headers.

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>