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

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.

Reviewed by: emaste, manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50994