SSL: cast hostname in SSL_set_tlsext_host_name().

BoringSSL promoted this macro to a proper function,
so it requires parameters with correct types now.

Change-Id: Iacc7bf8fa356c47f80ab6898cc1a8c340ea1d509
Signed-off-by: Piotr Sikora <piotrsikora@google.com>
Reviewed-on: https://nginx-review.googlesource.com/1006
Reviewed-by: Martin Maly <mmaly@google.com>