maint: address syntax-check errors.

* cfg.mk: Ignore the "error" call in tests/c++.at, it is not to be
translated.
* doc/bison.texi: Fix incorrect @pxref use.
* po/POTFILES.in: Add missing file.
* src/print_graph.c: Remove useless include.

cfg.mk

@@ -88,5 +88,5 @@ $(call exclude,								\
   require_config_h_first=^(lib/yyerror|data/(glr|yacc))\.c$$		\
   space_tab=^tests/(input|c\+\+)\.at$$					\
   trailing_blank=^src/parse-gram.[ch]$$					\
-  unmarked_diagnostics=^(djgpp/|doc/bison.texi$$)			\
+  unmarked_diagnostics=^(djgpp/|doc/bison.texi$$|tests/c\+\+\.at$$)	\
 )