style: rename print_graph.* as print-graph.*

These are the only files with _.

* src/print_graph.h, src/print_graph.c: Rename as...
* src/print-graph.h, src/print-graph.c: these.

src/print-graph.h

@@ -0,0 +1,26 @@
+/* Output a graph of the generated parser, for Bison.
+
+   Copyright (C) 2000, 2006, 2009-2015, 2018-2019 Free Software
+   Foundation, Inc.
+
+   This file is part of Bison, the GNU Compiler Compiler.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef PRINT_GRAPH_H_
+# define PRINT_GRAPH_H_
+
+void print_graph (void);
+
+#endif /* !PRINT_GRAPH_H_ */