MINOR: time: uninline report_idle() and move it to task.c

MINOR: time: uninline report_idle() and move it to task.c

I don't know why I inlined this one, this makes no sense given that it's
only used for stats, and it starts a circular dependency on tinfo.h which
can be problematic in the future. In addition, all the stuff related to
idle time calculation should be with the rest of the scheduler, which
currently is in task.{c,h}, so let's move it there.