i18n: also look in src/parse-gram.c

Some strings appears in the generated file only, e.g., translation of
"end of file".  So don't forget to go and see there.

po/POTFILES.in

@@ -8,6 +8,7 @@ src/graphviz.c
 src/location.c
 src/main.c
 src/muscle-tab.c
+src/parse-gram.c
 src/parse-gram.y
 src/print-graph.c
 src/print.c