openni_capture sample: enable OpenNI2 if available

Sample works with OpenNI2 with minimal code changes (CAP_OPENNI -> CAP_OPENNI2)

samples/cpp/openni_capture.cpp

@@ -192,7 +192,11 @@ int main( int argc, char* argv[] )
     if( isVideoReading )
         capture.open( filename );
     else
+    {
+        capture.open( CAP_OPENNI2 );
+        if( !capture.isOpened() )
             capture.open( CAP_OPENNI );
+    }
 
     cout << "done." << endl;