Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/gui/SurfaceComposerClient.cpp b/libs/gui/SurfaceComposerClient.cpp
index 6ad47d8..5863798 100644
--- a/libs/gui/SurfaceComposerClient.cpp
+++ b/libs/gui/SurfaceComposerClient.cpp
@@ -67,7 +67,7 @@
             mComposerService.composerServiceDied();
         }
      public:
-        DeathObserver(ComposerService& mgr) : mComposerService(mgr) { }
+        explicit DeathObserver(ComposerService& mgr) : mComposerService(mgr) { }
     };
 
     mDeathObserver = new DeathObserver(*const_cast<ComposerService*>(this));