libvncclient: tls_gnutls: Fix gnutls_transport_set_errno on Windows

gnutls_transport_set_errno takes a plain "gnutls_session_t", not
a "gnutls_session_t *".

This fixes build errors if building with a recent version of Clang
targeting Windows:

    libvncserver/src/libvncclient/tls_gnutls.c:161:32: error: incompatible pointer types passing 'gnutls_session_t *' (aka 'struct gnutls_session_int **') to parameter of type 'gnutls_session_t' (aka 'struct gnutls_session_int *'); dereference with * [-Wincompatible-pointer-types]
      161 |     gnutls_transport_set_errno(session, EAGAIN);
          |                                ^~~~~~~
          |                                *
    x86_64-w64-mingw32/include/gnutls/gnutls.h:2429:50: note: passing argument to parameter 'session' here
     2429 | void gnutls_transport_set_errno(gnutls_session_t session, int err);
          |                                                  ^