x86/mm: Include spinlock_t definition in pgtable.

x86/mm: Include spinlock_t definition in pgtable.

BugLink: https://bugs.launchpad.net/bugs/1972899
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: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20211102165224.wpz4zyhsvwccx5p3@linutronix.de
(cherry picked from commit 35fa745286ac44ee26ed100c2bd2553368ad193b)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Philip Cox <philip.cox@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>