Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: Ibdd6a210bb7ff228e3624cc319169f77aca3b51e
diff --git a/libs/storage/IMountService.cpp b/libs/storage/IMountService.cpp
index c643ed0..74638e7 100644
--- a/libs/storage/IMountService.cpp
+++ b/libs/storage/IMountService.cpp
@@ -55,7 +55,7 @@
 class BpMountService: public BpInterface<IMountService>
 {
 public:
-    BpMountService(const sp<IBinder>& impl)
+    explicit BpMountService(const sp<IBinder>& impl)
         : BpInterface<IMountService>(impl)
     {
     }