x86/cfi: Only define poison_cfi() if CONFIG_X86_KERNEL_IBT=y

poison_cfi() was introduced in:

  9831c625

 ("x86/cfi: Extend ENDBR sealing to kCFI")

... but it's only ever used under CONFIG_X86_KERNEL_IBT=y,
and if that option is disabled, we get:

  arch/x86/kernel/alternative.c:1243:13: error: ‘poison_cfi’ defined but not used [-Werror=unused-function]

Guard the definition with CONFIG_X86_KERNEL_IBT.

Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>