Include what you use: <cutils/bitops.h> for popcount

Test: builds OK
Change-Id: I46e27589936415dbf5da5722cd4132002c076c99
diff --git a/services/audioflinger/Threads.cpp b/services/audioflinger/Threads.cpp
index bf4195f..796596a 100644
--- a/services/audioflinger/Threads.cpp
+++ b/services/audioflinger/Threads.cpp
@@ -28,6 +28,7 @@
 #include <linux/futex.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
+#include <cutils/bitops.h>
 #include <cutils/properties.h>
 #include <media/AudioContainers.h>
 #include <media/AudioDeviceTypeAddr.h>