Add missing includes for Mutex.h

The missing header was being transitively included through a
Skia header which is being removed.

Change-Id: I5a65bae076027951fe07d1e7eecd4732124fb0e4
diff --git a/libs/hwui/TextureCache.h b/libs/hwui/TextureCache.h
index d879392..10a05e0 100644
--- a/libs/hwui/TextureCache.h
+++ b/libs/hwui/TextureCache.h
@@ -19,6 +19,7 @@
 
 #include <SkBitmap.h>
 
+#include <utils/Mutex.h>
 #include <utils/Vector.h>
 
 #include "Debug.h"