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/include/gui/ISurfaceComposer.h b/include/gui/ISurfaceComposer.h
index 9f1937b..d0031fb 100644
--- a/include/gui/ISurfaceComposer.h
+++ b/include/gui/ISurfaceComposer.h
@@ -38,7 +38,7 @@
 
 class ComposerState;
 class DisplayState;
-class DisplayInfo;
+struct DisplayInfo;
 class IDisplayEventConnection;
 class IMemoryHeap;
 class Rect;