Fix portable + mips build.

Change-Id: Ia200e582b04c84973281e12331777351feb8a401
diff --git a/runtime/monitor.h b/runtime/monitor.h
index d7de8a5..bfd8545 100644
--- a/runtime/monitor.h
+++ b/runtime/monitor.h
@@ -174,6 +174,9 @@
   Mutex monitor_lock_ DEFAULT_MUTEX_ACQUIRED_AFTER;
   ConditionVariable monitor_contenders_ GUARDED_BY(monitor_lock_);
 
+  // Number of people waiting on the condition.
+  size_t num_waiters_ GUARDED_BY(monitor_lock_);
+
   // Which thread currently owns the lock?
   Thread* volatile owner_ GUARDED_BY(monitor_lock_);