explicit_bzero-tests: pacify -Wmissing-declarations

[Bruno Haible]

Hi Paul,

> * tests/test-explicit_bzero.c (do_secret_stuff, test_stack):
>       Now static.

It's OK to mark 'test_stack' static. But for 'do_secret_stuff', it invites the
compiler to inline the function and then to do different optimizations for
pass == 1 and pass == 2, which will break the test.

Is it possible to portably mark 'do_secret_stuff' as not-inline? If not, then
I would prefer to revert the change and instead add a function declaration, to
avoid the warning.

Bruno