Clean up Fence a little.

Test: Cleanup patch only. Fences still work.

Change-Id: I65e5cdf5e3c067bc475c085ab6d3577d9753231c
diff --git a/libs/gui/IGraphicBufferProducer.cpp b/libs/gui/IGraphicBufferProducer.cpp
index e37b65b..18a80e8 100644
--- a/libs/gui/IGraphicBufferProducer.cpp
+++ b/libs/gui/IGraphicBufferProducer.cpp
@@ -20,6 +20,7 @@
 #include <utils/Errors.h>
 #include <utils/NativeHandle.h>
 #include <utils/RefBase.h>
+#include <utils/String8.h>
 #include <utils/Timers.h>
 #include <utils/Vector.h>