timer/arm_mptimer: Build arm_mptimer only once

Since current_cpu is CPUState it no longer depends on CPUARMState.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/hw/timer/arm_mptimer.c b/hw/timer/arm_mptimer.c
index fbc69c9..0ceb240 100644
--- a/hw/timer/arm_mptimer.c
+++ b/hw/timer/arm_mptimer.c
@@ -21,6 +21,7 @@
 
 #include "hw/sysbus.h"
 #include "qemu/timer.h"
+#include "qom/cpu.h"
 
 /* This device implements the per-cpu private timer and watchdog block
  * which is used in both the ARM11MPCore and Cortex-A9MP.