x86, watchdog: Fix build error in hw_nmi.c

On some configs the following build error triggers:

 arch/x86/kernel/apic/hw_nmi.c:35: error: 'apic' undeclared (first use in this function)
 arch/x86/kernel/apic/hw_nmi.c:35: error: (Each undeclared identifier is reported only once
 arch/x86/kernel/apic/hw_nmi.c:35: error: for each function it appears in.)

Because asm/apic.h was only included implicitly. Include it explicitly.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
LKML-Reference: <1273713674-8434-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>