ubsan: fix unused variable warning in test module

ubsan: fix unused variable warning in test module

This is one of the drivers with an unused variable that is marked 'const'.
Adding a __used annotation here avoids the warning and lets us enable
the option by default:

lib/test_ubsan.c:137:28: error: unused variable 'skip_ubsan_array' [-Werror,-Wunused-const-variable]

Fixes: 4a26f49b7b3d ("ubsan: expand tests and reporting")
Signed-off-by: Arnd Bergmann <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>