x86/resctrl: Include pid.h

We are about to disturb the header soup. This header uses struct pid
and struct pid_namespace. Include their header.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lkml.kernel.org/r/20200708163929.2783-6-james.morse@arm.com