Silence "initialization discards ‘const’ qualifier from pointer target type" warnings.

Silence "initialization discards ‘const’ qualifier from pointer target type" warnings.

* gettext-tools/src/pos.h (struct lex_pos_ty): Change type of file_name field to
'const char *'.
* gettext-tools/src/read-catalog.c (default_destructor,
default_reset_comment_state): Cast file_name to 'char *' before free()ing it.
* gettext-tools/src/xg-arglist-parser.h (arglist_parser_remember,
arglist_parser_remember_msgctxt): Change type of file_name parameter.
* gettext-tools/src/xg-arglist-parser.c (arglist_parser_remember,
arglist_parser_remember_msgctxt): Likewise.