Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I4504e98a8db31e0edcbe63c23f9af43eb13e9d86
diff --git a/fastboot/usb_linux.cpp b/fastboot/usb_linux.cpp
index d4824fb..6db1e27 100644
--- a/fastboot/usb_linux.cpp
+++ b/fastboot/usb_linux.cpp
@@ -89,7 +89,7 @@
 
 class LinuxUsbTransport : public Transport {
   public:
-    LinuxUsbTransport(std::unique_ptr<usb_handle> handle) : handle_(std::move(handle)) {}
+    explicit LinuxUsbTransport(std::unique_ptr<usb_handle> handle) : handle_(std::move(handle)) {}
     ~LinuxUsbTransport() override = default;
 
     ssize_t Read(void* data, size_t len) override;