kernel/timeout: Fix another recursive spinlock()

The fix in commit e664c78b ("kernel/timeout: Fix recursive
spinlock in z_set_timeout_expiry()") missed a spot that had also been
introduced with recent locking work.  The new
_get_next_timeout_expiry() implementation takes its own lock, which is
recursive when called from z_clock_announce().  Fix by calling the
wrapped implementation instead.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>