Clean up for warning:

Clean up for warning:
  fe-auth.c:573: warning: passing argument 1 of 'free' discards qualifiers
  from pointer target type
pg_krb5_authname used to return a (const char *) to memory allocated by
krb. Somewhere along the lines this was changed so that a copy was
made, returned, and freed instead. However the const modifier was never
removed.