kconfig: qconf: fix buffer overflow in debug links

If you enable "Option -> Show Debug Info" and click a link, the program
terminates with the following error:

    *** buffer overflow detected ***: terminated

The buffer overflow is caused by the following line:

    strcat(data, "$");

The buffer needs one more byte to accommodate the additional character.

Fixes: c4f7398b ("kconfig: qconf: make debug links work again")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>