x86, apic: merge genapic.h into apic.h

Impact: cleanup

Reduce the number of include files to worry about.
Also, most of the users of APIC facilities had to
include genapic.h already, which embedded apic.h,
so the distinction was meaningless.

[ include apic.h from genapic.h for compatibility. ]

Signed-off-by: Ingo Molnar <mingo@elte.hu>