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.orgSigned-off-by: Andreas Larsson <andreas@gaisler.com>

arch/sparc/kernel/setup_64.c

@@ -599,7 +599,7 @@ static void __init init_sparc64_elf_hwcap(void)
 		pause_patch();
 }
 
-void __init alloc_irqstack_bootmem(void)
+static void __init alloc_irqstack_bootmem(void)
 {
 	unsigned int i, node;