schannel: reorder functions topologically

The _shutdown() function calls the _session_free() function; While this
is not a problem now (because schannel.h declares both functions), a
patch looming in the immediate future with make all of these functions
file-local.

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

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