fix missing header for dump_model_graph (#2464)

fixes "invalid use of incomplete type lbann::model" when compiling with LBANN_HAS_BOOST

src/callbacks/dump_model_graph.cpp

@@ -25,6 +25,7 @@
 ////////////////////////////////////////////////////////////////////////////////
 
 #include "lbann/callbacks/dump_model_graph.hpp"
+#include "lbann/models/model.hpp"
 #include "lbann/utils/exception.hpp"
 #include "lbann_config.hpp"