schedstats/eas: guard properly to avoid breaking non-smp schedstats users

Add appropriate #ifdef guards to ensure the smp-only easstats structs
are not used when smp is not enabled. Arnd got a report from buildbot,
analysed it, and pointed out exactly what the issue was.

Reported-by: "Arnd Bergmann" <arnd@arndb.de>
Suggested-by: "Arnd Bergmann" <arnd@arndb.de>
Fixes: 4b85765a ("sched/fair: Add eas (& cas)
 specific rq, sd and task stats")
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Change-Id: I60554dea20137f6774db3f59b4afd40a06554cfc