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);
          |                                                  ^

Author: mstorsjo

src/libvncclient/tls_gnutls.c

@@ -253,7 +253,7 @@ InitializeTLS(void)
  * libvncclient are linked to different versions of msvcrt.dll.
  */
 #ifdef WIN32
-static void WSAtoTLSErrno(gnutls_session_t* session)
+static void WSAtoTLSErrno(gnutls_session_t session)
 {
   switch(WSAGetLastError()) {
 #if (GNUTLS_VERSION_NUMBER >= 0x029901)
@@ -293,7 +293,7 @@ PushTLS(gnutls_transport_ptr_t transport, const void *data, size_t len)
     if (ret < 0)
     {
 #ifdef WIN32
-      WSAtoTLSErrno((gnutls_session_t*)&client->tlsSession);
+      WSAtoTLSErrno((gnutls_session_t)client->tlsSession);
 #endif
       if (errno == EINTR) continue;
       return -1;
@@ -315,7 +315,7 @@ PullTLS(gnutls_transport_ptr_t transport, void *data, size_t len)
     if (ret < 0)
     {
 #ifdef WIN32
-      WSAtoTLSErrno((gnutls_session_t*)&client->tlsSession);
+      WSAtoTLSErrno((gnutls_session_t)client->tlsSession);
 #endif
       if (errno == EINTR) continue;
       return -1;
@@ -337,7 +337,7 @@ PullTimeout(gnutls_transport_ptr_t transport, unsigned int timeout)
     if (ret < 0)
     {
 #ifdef WIN32
-      WSAtoTLSErrno((gnutls_session_t*)&client->tlsSession);
+      WSAtoTLSErrno((gnutls_session_t)client->tlsSession);
 #endif
       if (errno == EINTR) continue;
     }