x86: surround apic headers in apic definitions

Although those constants are always defined in x86_64,
and will have the effect of just including the headers
in the very way we did before, I'm doing this in a separate
patch to be conservative and avoid surprises.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>