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/GradientCache.h b/libs/hwui/GradientCache.h
index 7339853..ac34684 100644
--- a/libs/hwui/GradientCache.h
+++ b/libs/hwui/GradientCache.h
@@ -19,6 +19,7 @@
 
 #include <SkShader.h>
 
+#include <utils/Mutex.h>
 #include <utils/Vector.h>
 
 #include "Texture.h"