Fix compilation error.

Introduced when #798 was merged.

Author: ndryden

src/proto/proto_common.cpp

@@ -4,6 +4,7 @@
 #include "lbann/base.hpp"
 #include "lbann/comm.hpp"
 #include "lbann/proto/init_image_data_readers.hpp"
+#include "lbann/utils/file_utils.hpp"
 
 #include <google/protobuf/io/coded_stream.h>
 #include <google/protobuf/io/zero_copy_stream_impl.h>