libc: fix the stubs for pthread_{suspend,resume}_all_np

Noticed just a little too late, stub_null returns a void * but these
prototypes have no return value. As far as I know, all of our archs
will throw the return value in a caller-saved register and it'll simply
be ignored, but it's probably worth being more accurate.

Fixes: 83aafcdc8892 ("libc, libthr: coordinate stubs for [...]")