BUILD: stats: conditionally mark obsolete stats states as deprecated

The obsolete stats states STAT_ST_* were marked as deprecated with recent
commit 6ef1648dc ("CLEANUP: stats: rename the stats state values an mark
the old ones deprecated"), except that this feature requires gcc 6 and
above. Let's use the macro that depends on this condition instead.

The issue appeared on 2.6-dev9 so no backport is needed.
1 file changed