Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I4504e98a8db31e0edcbe63c23f9af43eb13e9d86
diff --git a/gatekeeperd/IUserManager.cpp b/gatekeeperd/IUserManager.cpp
index 8645fc2..8167d19 100644
--- a/gatekeeperd/IUserManager.cpp
+++ b/gatekeeperd/IUserManager.cpp
@@ -27,7 +27,7 @@
 class BpUserManager : public BpInterface<IUserManager>
 {
 public:
-    BpUserManager(const sp<IBinder>& impl) :
+    explicit BpUserManager(const sp<IBinder>& impl) :
             BpInterface<IUserManager>(impl) {
     }
     virtual int32_t getCredentialOwnerProfile(int32_t user_id) {