Fix build with GLib >=2.68

Index: pv/data/decode/annotation.cpp
--- pv/data/decode/annotation.cpp.orig
+++ pv/data/decode/annotation.cpp
@@ -17,9 +17,7 @@
  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  */
 
-extern "C" {
 #include <libsigrokdecode/libsigrokdecode.h>
-}
 
 #include <cassert>
 #include <vector>
