Fix issues with clang and BUILD_HOST_64bit.

Change-Id: Id954d0c1144de6eaf89a4d27d205e3bf6ccb655f
diff --git a/runtime/monitor_pool.h b/runtime/monitor_pool.h
index 32f3f4e..82d0fee 100644
--- a/runtime/monitor_pool.h
+++ b/runtime/monitor_pool.h
@@ -17,12 +17,15 @@
 #ifndef ART_RUNTIME_MONITOR_POOL_H_
 #define ART_RUNTIME_MONITOR_POOL_H_
 
-#include "monitor.h"
-
-#include "safe_map.h"
-
+#ifdef __LP64__
+#include <bitset>
 #include <stdint.h>
 
+#include "monitor.h"
+#include "runtime.h"
+#include "safe_map.h"
+#endif
+
 namespace art {
 
 // Abstraction to keep monitors small enough to fit in a lock word (32bits). On 32bit systems the