x86/sev: Make boot_ghcb_page[] static

boot_ghcb_page is not used by any other file, so make it static.

This also resolves sparse warning:

  arch/x86/boot/compressed/sev.c:28:13: warning: symbol 'boot_ghcb_page' was not declared. Should it be static?

Signed-off-by: GUO Zihua <guozihua@huawei.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org