[Bugfix] Fix Windows build, recently added GNUTLS compat code broke it.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4279 3789f03b-4d11-0410-bbf8-ca57d06f2519
diff --git a/common/os/tls.cxx b/common/os/tls.cxx
index cd67ce7..c092996 100644
--- a/common/os/tls.cxx
+++ b/common/os/tls.cxx
@@ -32,7 +32,7 @@
 
 using namespace std;
 
-#ifdef HAVE_GNUTLS
+#if defined(HAVE_GNUTLS) && !defined(WIN32)
 #include <gnutls/gnutls.h>
 #include <gnutls/x509.h>