Fix google-explicit-constructor warnings in libhardware

Bug: 28341362
Change-Id: I29a56734e54740e4b15fe2f6cacbbd867163ae79
Test: build with clang-tidy
diff --git a/modules/usbcamera/UsbCamera.h b/modules/usbcamera/UsbCamera.h
index fe52ade..57017fe 100644
--- a/modules/usbcamera/UsbCamera.h
+++ b/modules/usbcamera/UsbCamera.h
@@ -28,7 +28,7 @@
  */
 class UsbCamera : public Camera {
     public:
-        UsbCamera(int id);
+        explicit UsbCamera(int id);
         ~UsbCamera();
 
     private: