arm64/spinlock: fix a -Wunused-function warning

The commit f5bfdc8e ("locking/osq: Use optimized spinning loop for
arm64") introduced a warning from Clang because vcpu_is_preempted() is
compiled away,

kernel/locking/osq_lock.c:25:19: warning: unused function 'node_cpu'
[-Wunused-function]
static inline int node_cpu(struct optimistic_spin_node *node)
                  ^
1 warning generated.

Fix it by converting vcpu_is_preempted() to a static inline function.

Fixes: f5bfdc8e ("locking/osq: Use optimized spinning loop for arm64")
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/spinlock.h

@@ -18,6 +18,10 @@
  * See:
  * https://lore.kernel.org/lkml/20200110100612.GC2827@hirez.programming.kicks-ass.net
  */
-#define vcpu_is_preempted(cpu)	false
+#define vcpu_is_preempted vcpu_is_preempted
+static inline bool vcpu_is_preempted(int cpu)
+{
+	return false;
+}
 
 #endif /* __ASM_SPINLOCK_H */