kconfig: keep config.gz around even if CONFIG_IKCONFIG_PROC is not set

If CONFIG_IKCONFIG is set but CONFIG_IKCONFIG_PROC is not, then
gcc will optimize the config.gz out, because nobody uses it.

This patch adds "__used" to the config.gz data to keep it around so that
code like extract-ikconfig can still find it.

[ Impact: allow extract-ikconfig to find config.gz ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>