Fix build error with GCC 4.8.1

[kou]

This fixes "format not a string literal and no format arguments [-Werror=format-security]" error:

conn.c: In function ‘rb_ldap_conn_start_tls_s’:
conn.c:221:3: error: format not a string literal and no format arguments [-Werror=format-security]
   Check_LDAP_Result (ldapdata->err);
   ^
....
conn.c:557:3: error: format not a string literal and no format arguments [-Werror=format-security]
   Check_LDAP_OPT_Result (ldapdata->err);
   ^
....
conn.c: In function ‘rb_ldap_conn_get_option’:
conn.c:703:7: error: format not a string literal and no format arguments [-Werror=format-security]
       rb_raise (rb_eLDAP_Error, ldap_err2string (ldapdata->err));
       ^

[ghost]

Thank you, @kou !