Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/gui/IGraphicBufferProducer.cpp b/libs/gui/IGraphicBufferProducer.cpp
index 1099c84..e91a8e1 100644
--- a/libs/gui/IGraphicBufferProducer.cpp
+++ b/libs/gui/IGraphicBufferProducer.cpp
@@ -54,7 +54,7 @@
 class BpGraphicBufferProducer : public BpInterface<IGraphicBufferProducer>
 {
 public:
-    BpGraphicBufferProducer(const sp<IBinder>& impl)
+    explicit BpGraphicBufferProducer(const sp<IBinder>& impl)
         : BpInterface<IGraphicBufferProducer>(impl)
     {
     }