Fix issues with clang and BUILD_HOST_64bit.

Change-Id: Id954d0c1144de6eaf89a4d27d205e3bf6ccb655f
diff --git a/runtime/monitor_pool.cc b/runtime/monitor_pool.cc
index 19e569d..eb7525a 100644
--- a/runtime/monitor_pool.cc
+++ b/runtime/monitor_pool.cc
@@ -18,6 +18,7 @@
 
 #include "base/logging.h"
 #include "base/mutex-inl.h"
+#include "thread-inl.h"
 #include "monitor.h"
 
 namespace art {