Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I7e061f68acdde368a3b63c029b928133646ebff2
diff --git a/libs/binder/IResultReceiver.cpp b/libs/binder/IResultReceiver.cpp
index 2a22b69..646809e 100644
--- a/libs/binder/IResultReceiver.cpp
+++ b/libs/binder/IResultReceiver.cpp
@@ -31,7 +31,7 @@
 class BpResultReceiver : public BpInterface<IResultReceiver>
 {
 public:
-    BpResultReceiver(const sp<IBinder>& impl)
+    explicit BpResultReceiver(const sp<IBinder>& impl)
         : BpInterface<IResultReceiver>(impl)
     {
     }