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

Test: builds OK
Change-Id: I46e27589936415dbf5da5722cd4132002c076c99
diff --git a/services/audioflinger/FastMixer.cpp b/services/audioflinger/FastMixer.cpp
index 3eacc8c..bf2e953 100644
--- a/services/audioflinger/FastMixer.cpp
+++ b/services/audioflinger/FastMixer.cpp
@@ -40,6 +40,7 @@
 #include <audio_utils/channels.h>
 #include <audio_utils/format.h>
 #include <audio_utils/mono_blend.h>
+#include <cutils/bitops.h>
 #include <media/AudioMixer.h>
 #include "FastMixer.h"
 #include "TypedLogger.h"