mailbox: mailbox-test: Explicitly include header for spinlock support

Presently the support appears to be implied.
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox-test.c

@@ -16,6 +16,7 @@
 #include <linux/platform_device.h>
 #include <linux/poll.h>
 #include <linux/slab.h>
+#include <linux/spinlock.h>
 #include <linux/uaccess.h>
 #include <linux/sched/signal.h>