DECLARE_PER_CPU needs linux/percpu.h

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 508ce20..1e6f731 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -20,6 +20,8 @@
 #include <linux/init.h>
 #include <linux/list.h>
 #include <linux/wait.h>
+#include <linux/percpu.h>
+
 
 struct hrtimer_clock_base;
 struct hrtimer_cpu_base;