Include what you use

Change-Id: I1967861f383bb5ed6743fb69e3bd439907ed7033
diff --git a/services/audioflinger/Tracks.cpp b/services/audioflinger/Tracks.cpp
index 7757ea2..78cec31 100644
--- a/services/audioflinger/Tracks.cpp
+++ b/services/audioflinger/Tracks.cpp
@@ -20,6 +20,7 @@
 //#define LOG_NDEBUG 0
 
 #include "Configuration.h"
+#include <linux/futex.h>
 #include <math.h>
 #include <sys/syscall.h>
 #include <utils/Log.h>