x86/ioapic: Mark mp_alloc_timer_irq() __init

Only invoked from check_timer() which is __init too. Cleanup the variable
declaration while at it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Tested-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/all/20240802155440.339321108@linutronix.de