Add a missing visibility modifier to fix build.

This class is used by the tests, but is built with -fvisibility=hidden
and not declared as ANDROID_API.

Change-Id: If93af7bfdf0752de1f5bbb10fdd46b2eb9c0ccdd
diff --git a/libs/hwui/CanvasState.h b/libs/hwui/CanvasState.h
index b15698e..6883d0a4 100644
--- a/libs/hwui/CanvasState.h
+++ b/libs/hwui/CanvasState.h
@@ -70,7 +70,7 @@
  * that quickRejection can also be used.
  */
 
-class CanvasState {
+class ANDROID_API CanvasState {
 public:
     CanvasState(CanvasStateClient& renderer);
     ~CanvasState();