Build error SensorManager.

Remove forward declaration for String8 and inlcude the corresponding header file.

Change-Id: I89a4ea42295dc9bb6db2c304ff725cc1d498cd55
diff --git a/include/gui/SensorManager.h b/include/gui/SensorManager.h
index 7d43273..1afff68 100644
--- a/include/gui/SensorManager.h
+++ b/include/gui/SensorManager.h
@@ -26,6 +26,7 @@
 #include <utils/RefBase.h>
 #include <utils/Singleton.h>
 #include <utils/Vector.h>
+#include <utils/String8.h>
 
 #include <gui/SensorEventQueue.h>
 
@@ -40,7 +41,6 @@
 class ISensorServer;
 class Sensor;
 class SensorEventQueue;
-class String8;
 // ----------------------------------------------------------------------------
 
 class SensorManager :