surfaceflinger: fix -Wmismatched-tags warnings

warning: struct 'HWComposer' was previously declared as a class
[-Wmismatched-tags]

warning: class 'DisplayInfo' was previously declared as a struct
[-Wmismatched-tags]

Change-Id: I13db9f8aab3a957ce8ff8d64598dae1807d6fe7e
diff --git a/services/surfaceflinger/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index d3f784a..8695a44 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;