sparc64: Fix prototype warning for alloc_irqstack_bootmem

Fix the following warning:
arch/sparc/kernel/setup_64.c:602:13: warning: no previous prototype for ‘alloc_irqstack_bootmem’

The function alloc_irqstack_bootmem had no users outside setup_64.c so
declare it static.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: David S. Miller <davem@davemloft.net>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Tested-by: Andreas Larsson <andreas@gaisler.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240330-sparc64-warnings-v1-4-37201023ee2f@ravnborg.org
Signed-off-by: Andreas Larsson <andreas@gaisler.com>