MXS-1354: Remove external linkage from User class

The User class is no longer declared with external linkage.

server/core/users.cc

@@ -20,6 +20,8 @@
 #include <maxscale/users.h>
 #include <maxscale/authenticator.h>
 
+namespace
+{
 class Users
 {
     Users(const Users&);
@@ -108,6 +110,8 @@ private:
     UserMap m_data;
 };
 
+}
+
 USERS *users_alloc()
 {
     Users* rval = new (std::nothrow) Users();