Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/radio/IRadio.cpp b/radio/IRadio.cpp
index 242df77..0881a91 100644
--- a/radio/IRadio.cpp
+++ b/radio/IRadio.cpp
@@ -44,7 +44,7 @@
 class BpRadio: public BpInterface<IRadio>
 {
 public:
-    BpRadio(const sp<IBinder>& impl)
+    explicit BpRadio(const sp<IBinder>& impl)
         : BpInterface<IRadio>(impl)
     {
     }