kconfig: remove unused variable in qconf.cc

If this file were compiled with -Wall, the following warning would be
reported:

scripts/kconfig/qconf.cc:312:6: warning: unused variable ‘i’ [-Wunused-variable]
  int i;
      ^

The commit prepares to turn on -Wall for C++ host programs.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>