pgsql: Clean up for warning: fe-auth.c:573: warning: passing argument

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/interfaces/libpq:
fe-auth.c (r1.115 -> r1.116)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.115&r2=1.116)