async_win.c: remove unused variable

Silencing this:
    crypto/async/arch/async_win.c: In function 'async_fibre_init_dispatcher':
    crypto/async/arch/async_win.c:112:12: warning: unused variable 'dispatcher' [-Wunused-variable]
         LPVOID dispatcher;
                ^

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>