Fix clang warnings about struct vs. class.

Change-Id: I9f26cd01ac3153efa6d77a4133686842e2d63203
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index 9f96113..30df461 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -191,7 +191,7 @@
      * This behaves more or less like a forward iterator.
      */
     class LayerListIterator {
-        friend struct HWComposer;
+        friend class HWComposer;
         HWCLayer* const mLayerList;
         size_t mIndex;