x86/apic: Hide unused safe_smp_processor_id() on 32-bit UP

When CONFIG_SMP is disabled in a 32-bit config, the prototype for
safe_smp_processor_id() is hidden, which causes a W=1 warning:

  arch/x86/kernel/apic/ipi.c:316:5: error: no previous prototype for 'safe_smp_processor_id' [-Werror=missing-prototypes]

Since there are no callers in this configuration, just hide the definition
as well.

  [ bp: Clarify it is a 32-bit config. ]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230725134837.1534228-2-arnd@kernel.org