kconfig: fix off-by-one in zconf_error()

yyerror() reports the line number of the next line.

This +1 adjustment was introduced more than 20 years ago [1]. At that
time, the line number was decremented then incremented back and forth.

The line number management was refactored in a more maintainable way.
Such compensation is no longer needed.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=d4f8a4530eb07a1385fd17b0e62a7dce97486f49



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>