arm: include definition for cpumask_t

This definition gets pulled in by other files. With the (later) split of
RCU and spinlock.h it won't compile anymore.
The split is done in ("rbtree: don't include the rcu header").

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

arch/arm/include/asm/irq.h

@@ -23,6 +23,8 @@
 #endif
 
 #ifndef __ASSEMBLY__
+#include <linux/cpumask.h>
+
 struct irqaction;
 struct pt_regs;