Add const to _kdc_set_e_text()
On Ubuntu 20.04 with gcc version 9.3.0 in a Samba compile
using -Wdiscarded-qualifiers and -Werror we get:
../../source4/heimdal/kdc/kerberos5.c:2516:21: warning: passing argument 2 of ‘_kdc_set_e_text’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
2516 | _kdc_set_e_text(r, "Client have no reply key");
| ^~~~~~~~~~~~~~~~~~~~~~~~~~
../../source4/heimdal/kdc/kerberos5.c:428:42: note: expected ‘char *’ but argument is of type ‘const char *’
428 | _kdc_set_e_text(astgs_request_t r, char *fmt, ...)
| ~~~~~~^~~
Signed-off-by: Andrew Bartlett <abartlet@samba.org>