locking/lockdep: Reduce header dependencies in <linux/debug_locks.h>

The inclusion of printk.h leads to a circular dependency if spinlock_t is
based on rtmutexes on RT enabled kernels.

Include only atomic.h (xchg()) and cache.h (__read_mostly) which is all
what debug_locks.h requires.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211303.484161136@linutronix.de