x86/mm: Include spinlock_t definition in pgtable.

This header file provides forward declartion for pgd_lock but does not
include the header defining its type. This works since the definition of
spinlock_t is usually included somehow via printk.

By trying to avoid recursive includes on PREEMPT_RT I avoided the loop
in printk and as a consequnce kernel/intel.c failed to compile due to
missing type definition.

Include the needed definition for spinlock_t.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211129174654.668506-11-bigeasy@linutronix.de