Add missing includes for threads.h

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

Change-Id: I5a65bae076027951fe07d1e7eecd4732124fb0e4

Author: drWulf

libs/hwui/GradientCache.h

@@ -19,6 +19,7 @@
 
 #include <SkShader.h>
 
+#include <utils/threads.h>
 #include <utils/Vector.h>
 
 #include "Texture.h"

libs/hwui/TextureCache.h

@@ -19,6 +19,7 @@
 
 #include <SkBitmap.h>
 
+#include <utils/threads.h>
 #include <utils/Vector.h>
 
 #include "Debug.h"