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).