Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I4504e98a8db31e0edcbe63c23f9af43eb13e9d86
diff --git a/fastboot/tcp.cpp b/fastboot/tcp.cpp
index e42c4e1..dd6fbf8 100644
--- a/fastboot/tcp.cpp
+++ b/fastboot/tcp.cpp
@@ -66,7 +66,7 @@
     int Close() override;
 
   private:
-    TcpTransport(std::unique_ptr<Socket> sock) : socket_(std::move(sock)) {}
+    explicit TcpTransport(std::unique_ptr<Socket> sock) : socket_(std::move(sock)) {}
 
     // Connects to the device and performs the initial handshake. Returns false and fills |error|
     // on failure.