x86/smp: fix non-SMP broken build due to redefinition of apic_id_is_primary_thread

The function has an inline "return false;" definition with CONFIG_SMP=n
but the "real" definition is also visible leading to "redefinition of
‘apic_id_is_primary_thread’" compiler error.

Guard it with #ifdef CONFIG_SMP

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Fixes: 6a4d2657 ("x86/smp: Provide topology_is_primary_thread()")
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>