Fix compilation error: '_1' was not declared in this scope

Change-Id: I0f1f2df817a65f7d5b72c5280451346a0a011d47
diff --git a/lib/transmitter/preprocess_tx_burst_impl.cc b/lib/transmitter/preprocess_tx_burst_impl.cc
index 5fbac98..2023ee6 100644
--- a/lib/transmitter/preprocess_tx_burst_impl.cc
+++ b/lib/transmitter/preprocess_tx_burst_impl.cc
@@ -55,7 +55,7 @@
         message_port_register_out(pmt::mp("bursts_out"));
 
         set_msg_handler(pmt::mp("bursts_in"),
-          boost::bind(&preprocess_tx_burst_impl::process_burst, this, _1));
+          boost::bind(&preprocess_tx_burst_impl::process_burst, this, boost::placeholders::_1));
     }
 
     /*