xen/arm: traps: Mark check_stack_alignment_constraints as unused

xen/arm: traps: Mark check_stack_alignment_constraints as unused

Clang will throw an error if a function is unused unless you tell
to ignore it. This can be done using __maybe_unused.

While modifying the declaration, update it to match prototype of similar
functions (see build_assertions). This helps to understand that the sole
purpose of the function is to hold BUILD_BUG_ON().

Signed-off-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>