x86/stackprotector/64: Only export __ref_stack_chk_guard on CONFIG_SMP

x86/stackprotector/64: Only export __ref_stack_chk_guard on CONFIG_SMP

[ Upstream commit 91d5451d97ce35cbd510277fa3b7abf9caa4e34d ]

The __ref_stack_chk_guard symbol doesn't exist on UP:

  <stdin>:4:15: error: ‘__ref_stack_chk_guard’ undeclared here (not in a function)

Fix the #ifdef around the entry.S export.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20250123190747.745588-8-brgerst@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>