tick/broadcast: Fix warning about undefined tick_broadcast_oneshot_offline()

Randconfig builds with

  CONFIG_TICK_ONESHOT=y
  CONFIG_HOTPLUG_CPU=n

trigger

  kernel/time/tick-broadcast.c:39:13: warning: ‘tick_broadcast_oneshot_offline’ \
	  declared ‘static’ but never defined [-Wunused-function]

due to that function's definition missing.

Move the CONFIG_HOTPLUG_CPU ifdeffery around its declaration too.

Fixes: 1b72d432 ("tick: Remove outgoing CPU from broadcast masks")
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20190329110508.6621-1-bp@alien8.de