Use internal implementation of logwtmp function to prevent
utilities from sys-apps/coreutils to not detect authorized users

Patch by: Sergey Popov <pinkbyte@gentoo.org>

--- ./pppd/sys-linux.c     2014-06-16 16:45:48.447810715 +0400
+++ ./pppd/sys-linux.c  2014-06-16 16:47:10.614805857 +0400
@@ -2196,7 +2197,6 @@
     return ok;
 }
 
-#ifndef HAVE_LOGWTMP
 /********************************************************************
  *
  * Update the wtmp file with the appropriate user name and tty device.
@@ -2270,7 +2270,7 @@
     }
 #endif
 }
-#endif /* HAVE_LOGWTMP */
+