trivial: Fix a compile failure with new GLib versions

GLib now 'helpfully' includes type_traits in gmacros.h -- which now explodes
when compiling with a C++ compiler.

contrib/ci/qt5-thread-test/qt-thread-test.cpp

@@ -4,9 +4,7 @@
  * SPDX-License-Identifier: LGPL-2.1+
  */
 
-extern "C" {
 #include <fwupd.h>
-}
 
 #include <QCoreApplication>
 #include <QtConcurrentRun>

libfwupd/fwupd-client-sync.h

@@ -8,6 +8,8 @@
 
 #include "fwupd-client.h"
 
+G_BEGIN_DECLS
+
 gboolean	 fwupd_client_connect			(FwupdClient	*self,
 							 GCancellable	*cancellable,
 							 GError		**error)
@@ -225,3 +227,5 @@ GBytes		*fwupd_client_upload_bytes		(FwupdClient	*self,
 							 GCancellable	*cancellable,
 							 GError		**error)
 							 G_GNUC_WARN_UNUSED_RESULT;
+
+G_END_DECLS