Fix clang warnings about struct vs. class.

Change-Id: I9f26cd01ac3153efa6d77a4133686842e2d63203
diff --git a/services/surfaceflinger/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index 01a9d2e..46292a4 100644
--- a/services/surfaceflinger/DisplayDevice.h
+++ b/services/surfaceflinger/DisplayDevice.h
@@ -37,7 +37,7 @@
 
 namespace android {
 
-class DisplayInfo;
+struct DisplayInfo;
 class DisplaySurface;
 class IGraphicBufferProducer;
 class Layer;