Fix google-explicit-constructor warnings in system/vold.

Bug: 28341362
Change-Id: I14344b0cba134570bf87e7d61aa03503735dd794
Test: build with WITH_TIDY=1
diff --git a/NetlinkHandler.h b/NetlinkHandler.h
index 00a31c8..56eb23c 100644
--- a/NetlinkHandler.h
+++ b/NetlinkHandler.h
@@ -22,7 +22,7 @@
 class NetlinkHandler: public NetlinkListener {
 
 public:
-    NetlinkHandler(int listenerSocket);
+    explicit NetlinkHandler(int listenerSocket);
     virtual ~NetlinkHandler();
 
     int start(void);