stack: Declare {randomize_,}kstack_offset to fix Sparse warnings

Fix the following Sparse warnings that got noticed when the PPC-dev
patchwork was checking another patch (see the link below):

init/main.c:862:1: warning: symbol 'randomize_kstack_offset' was not declared. Should it be static?
init/main.c:864:1: warning: symbol 'kstack_offset' was not declared. Should it be static?

Which in fact are triggered on all architectures that have
HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET support (for instances x86, arm64
etc).

Link: https://lore.kernel.org/lkml/[email protected]/T/#m49b2d4490121445ce4bf7653500aba59eefcb67f
Cc: Christophe Leroy <[email protected]>
Cc: Xiu Jianfeng <[email protected]>
Signed-off-by: GONG, Ruiqi <[email protected]>
Reviewed-by: Christophe Leroy <[email protected]>
Fixes: 39218ff ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: GONG, Ruiqi

init/main.c

@@ -99,6 +99,7 @@
 #include <linux/kcsan.h>
 #include <linux/init_syscalls.h>
 #include <linux/stackdepot.h>
+#include <linux/randomize_kstack.h>
 #include <net/net_namespace.h>
 
 #include <asm/io.h>