Fix the prototype of the stub function in noshare.c.

It's an old prototype from part way through the development of
connection sharing, which I must have forgotten to fix because by the
time I changed the prototype no platform was using noshare.c any more.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10183 cda61777-01e9-0310-a592-d414129be87e