libgettextpo: Fix compilation error on Android.
* gettext-tools/libgettextpo/gettext-po.c: Include error.h second.
(orig_error, orig_error_at_line): New variables.
(error, error_at_line): Undefine afterwards.
(po_file_read_v2, po_file_write, po_error_logger, po_message_check_format): Use
these variables instead of the error, error_at_line symbols.
