sched/fair.c: fix kernel-doc notation warning

Fix kernel-doc warning for missing 'flags' parameter description:

../kernel/sched/fair.c:3371: warning: Function parameter or member 'flags' not described in 'attach_entity_load_avg'

Fixes: ea14b57e ("sched/cpufreq: Provide migration hint")

Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/cdda0d42-880d-4229-a9f7-5899c977a063@infradead.org