Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I4504e98a8db31e0edcbe63c23f9af43eb13e9d86
diff --git a/fastboot/udp.cpp b/fastboot/udp.cpp
index b36bd60..53fb347 100644
--- a/fastboot/udp.cpp
+++ b/fastboot/udp.cpp
@@ -111,7 +111,7 @@
     int Close() override;
 
   private:
-    UdpTransport(std::unique_ptr<Socket> socket) : socket_(std::move(socket)) {}
+    explicit UdpTransport(std::unique_ptr<Socket> socket) : socket_(std::move(socket)) {}
 
     // Performs the UDP initialization procedure. Returns true on success.
     bool InitializeProtocol(std::string* error);