Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/radio/IRadioService.cpp b/radio/IRadioService.cpp
index 8c2b3ef..341fb33 100644
--- a/radio/IRadioService.cpp
+++ b/radio/IRadioService.cpp
@@ -45,7 +45,7 @@
 class BpRadioService: public BpInterface<IRadioService>
 {
 public:
-    BpRadioService(const sp<IBinder>& impl)
+    explicit BpRadioService(const sp<IBinder>& impl)
         : BpInterface<IRadioService>(impl)
     {
     }