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: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/async/arch/async_win.c

@@ -109,8 +109,6 @@ void async_global_cleanup(void)
 
 int async_fibre_init_dispatcher(async_fibre *fibre)
 {
-    LPVOID dispatcher;
-
     fibre->fibre = ConvertThreadToFiber(NULL);
     if (fibre->fibre == NULL) {
         fibre->converted = 0;