Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I4504e98a8db31e0edcbe63c23f9af43eb13e9d86
diff --git a/fastboot/socket.cpp b/fastboot/socket.cpp
index 14ecd93..e56ffcf 100644
--- a/fastboot/socket.cpp
+++ b/fastboot/socket.cpp
@@ -167,7 +167,7 @@
 // Implements the Socket interface for TCP.
 class TcpSocket : public Socket {
   public:
-    TcpSocket(cutils_socket_t sock) : Socket(sock) {}
+    explicit TcpSocket(cutils_socket_t sock) : Socket(sock) {}
 
     bool Send(const void* data, size_t length) override;
     bool Send(std::vector<cutils_socket_buffer_t> buffers) override;