auth/Auth.h: mark constructor as explicit

auth/Auth.h: mark constructor as explicit

Fix for cppcheck warning:
 Struct 'AuthAuthorizer' has a constructor with 1 argument that is
 not explicit. Such constructors should in general be explicit for
 type safety reasons. Using the explicit keyword in the constructor
 means some mistakes when using the class can be avoided.

For more information check:
http://www.tantalon.com/pete/cppopt/asyougo.htm#UseExplicitConstructors

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>