AD: Suppress warning Wincompatible-pointer-types with sasl callbacks
    
    SASL use different prototype for callbacks based on id.
    However struct sasl_callback_t contains generic callback int (*)(void)
    which is not compatible with these callbacks and gcc8 warns about it.
    
    src/providers/ad/ad_init.c:116:23: warning: cast between incompatible
        function types from ‘int (*)(void *, const char *, const char *,
                                     const char **, unsigned int *)’
        to ‘int (*)(void)’ [-Wcast-function-type]
         { SASL_CB_GETOPT, (sss_sasl_gen_cb_fn)ad_sasl_getopt, NULL },
                           ^
    src/providers/ad/ad_init.c:117:20: warning: cast between incompatible
        function types from ‘int (*)(void *, int,  const char *)’
        to ‘int (*)(void)’ [-Wcast-function-type]
         { SASL_CB_LOG, (sss_sasl_gen_cb_fn)ad_sasl_log, NULL },
                        ^
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    (cherry picked from commit 1b6965fd09e4e6a6b5ba76b8221ca3980bcc56b4)