powerpc/smp: Add __init to init_big_cores()

It fixes this link warning:

WARNING: modpost: vmlinux.o(.text.unlikely+0x2d98): Section mismatch in reference from the function init_big_cores.isra.0() to the function .init.text:init_thread_group_cache_map()
The function init_big_cores.isra.0() references
the function __init init_thread_group_cache_map().
This is often because init_big_cores.isra.0 lacks a __init
annotation or the annotation of init_thread_group_cache_map is wrong.

Fixes: 425752c ("powerpc: Detect the presence of big-cores via "ibm, thread-groups"")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201221074154.403779-1-clg@kaod.org

arch/powerpc/kernel/smp.c

@@ -990,7 +990,7 @@ static struct sched_domain_topology_level powerpc_topology[] = {
 	{ NULL, },
 };
 
-static int init_big_cores(void)
+static int __init init_big_cores(void)
 {
 	int cpu;