Avoid a compiler warning on some versions of gcc by using real
function prototypes.

Reported-by: Rob Shelley <Rob@cirris.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

All axTLS files are now vanilla versions of the upstream axTLS files,
with one minor exception: the unused "ctx" parameter of
bi_int_divide() has been marked with "__unused" to avoid a compilation
error.

Signed-off-by: Michael Brown <mcb30@ipxe.org>