axtls: reorder functions topologically

The connect_finish() function (like many other functions after it) calls
the Curl_axtls_close() function; While this is not a problem now
(because axtls.h declares the latter function), a patch looming in the
immediate future with make all of these functions file-local.

So let's just move the Curl_axtls_close() function's definition before
it is called.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>