Remove unnecessary extern "C"s, fix build failures.

Wrapping libsigrockdecode.h in extern "C" is unecessary and will cause
build failures on modern versions of glib, this fixes those failures.

Author: nickelpro

DSView/pv/data/decode/annotation.cpp

@@ -19,9 +19,7 @@
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
  */
 
-extern "C" {
 #include <libsigrokdecode4DSL/libsigrokdecode.h>
-}
 
 #include <vector>
 #include <assert.h>

DSView/pv/view/decodetrace.cpp

@@ -19,9 +19,7 @@
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
  */
 
-extern "C" {
 #include <libsigrokdecode4DSL/libsigrokdecode.h>
-}
 
 #include <extdef.h>
 

DSView/pv/widgets/decodergroupbox.cpp

@@ -18,9 +18,7 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
  */
-extern "C" {
 #include <libsigrokdecode4DSL/libsigrokdecode.h>
-}
 
 #include "decodergroupbox.h"
 #include "../data/decoderstack.h"