Fix build error with GCC 4.8.1

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));
           ^
Merge pull request #23 from kou/fix-build-error

Fix build error with GCC 4.8.1