Fix clang warnings about struct vs. class.

Change-Id: I9f26cd01ac3153efa6d77a4133686842e2d63203
diff --git a/include/gui/ISurfaceComposer.h b/include/gui/ISurfaceComposer.h
index 5c3c99c..ffc44c7 100644
--- a/include/gui/ISurfaceComposer.h
+++ b/include/gui/ISurfaceComposer.h
@@ -35,7 +35,7 @@
 
 class ComposerState;
 class DisplayState;
-class DisplayInfo;
+struct DisplayInfo;
 class IDisplayEventConnection;
 class IMemoryHeap;