linuxkpi: Include <linux/spinlock.h> from <linux/refcount.h>

[dumbbell]

Summary

The <linux/refcount.h> header uses the spinlock_t type and some consumers in the DRM drivers don't include <linux/spinlock.h> directly or indirectly. This led to compilation errors because the type was undefined.

On Linux, <linux/refcount.h> includes <linux/spinlock_type.h> as a comparision.

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