Add missing MAX_STACKDEPTH define on MUTEX_DEBUGGING build

Corrected a small oversight when locking related code was moved from
src/lxc/utils.c to src/lxc/lxclock.c.

Signed-off-by: Stephen M Bennett <stephen_m_bennett@hotmail.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>