Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/radio/IRadioClient.cpp b/radio/IRadioClient.cpp
index 033ca49..ca21949 100644
--- a/radio/IRadioClient.cpp
+++ b/radio/IRadioClient.cpp
@@ -33,7 +33,7 @@
 {
 
 public:
-    BpRadioClient(const sp<IBinder>& impl)
+    explicit BpRadioClient(const sp<IBinder>& impl)
         : BpInterface<IRadioClient>(impl)
     {
     }