Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/input/IInputFlinger.cpp b/libs/input/IInputFlinger.cpp
index e009731..003e73d 100644
--- a/libs/input/IInputFlinger.cpp
+++ b/libs/input/IInputFlinger.cpp
@@ -28,7 +28,7 @@
 
 class BpInputFlinger : public BpInterface<IInputFlinger> {
 public:
-    BpInputFlinger(const sp<IBinder>& impl) :
+    explicit BpInputFlinger(const sp<IBinder>& impl) :
             BpInterface<IInputFlinger>(impl) { }
 
     virtual status_t doSomething() {