bug: Have __warn() prototype defined unconditionally

The __warn() prototype is declared in CONFIG_BUG scope but the function
definition in panic.c is unconditional. The IBT enablement started using
it unconditionally but a CONFIG_X86_KERNEL_IBT=y, CONFIG_BUG=n .config
will trigger a

  arch/x86/kernel/traps.c: In function ‘__exc_control_protection’:
  arch/x86/kernel/traps.c:249:17: error: implicit declaration of function \
  	  ‘__warn’; did you mean ‘pr_warn’? [-Werror=implicit-function-declaration]

Pull up the declarations so that they're unconditionally visible too.

  [ bp: Rewrite commit message. ]

Fixes: 991625f3 ("x86/ibt: Add IBT feature, MSR and #CP handling")
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Shida Zhang <zhangshida@kylinos.cn>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220426032007.510245-1-starzhangzsd@gmail.com