Remove redeclaration of ttyname() in ksu

ttyname() is declared in unistd.h.  Redeclaring it causes an
issue building with clang fortify on Chrome OS.

https://github.com/krb5/krb5/commit/6ee24f97b98f9c045632b1ce0670a3eb508cb833
Author: Zentaro Kavanagh <zentaro@google.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 6ee24f97b98f9c045632b1ce0670a3eb508cb833
Branch: master
 src/clients/ksu/main.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)