[PATCH] Fix next_timer_interrupt() for hrtimer

[PATCH] Fix next_timer_interrupt() for hrtimer

This patch adds support for hrtimer to next_timer_interrupt()
and fixes current breakage.

Function next_timer_interrupt() got broken with a recent patch
6ba1b91213e81aa92b5cf7539f7d2a94ff54947c as sys_nanosleep() was
moved to hrtimer. This broke things as next_timer_interrupt()
did not check hrtimer tree for next event.

Function next_timer_interrupt() is needed with dyntick
(CONFIG_NO_IDLE_HZ, VST) implementations, as the system can
be in idle when next hrtimer event was supposed to happen.
At least ARM and S390 currently use next_timer_interrupt().

Signed-off-by: Tony Lindgren <tony@atomide.com>