x86, mpparse: Move check_slot into CONFIG_X86_IO_APIC context

When CONFIG_X86_MPPARSE=y and CONFIG_X86_IO_APIC=n, then we get
the following warning:

  arch/x86/kernel/mpparse.c:723: warning: 'check_slot' defined but not used

So, put check_slot into CONFIG_X86_IO_APIC context. Its only
called from CONFIG_X86_IO_APIC=y context.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
LKML-Reference: <AANLkTinsUfGc=NG_GeH_B+zFVu+DXJzZbJKdQLscqfuH@mail.gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>