libwbclient: Fix warning statement with no effect
    
    src/sss_client/libwbclient/wbc_pam_sssd.c: In function ‘wbcAuthenticateUserEx’:
    src/sss_client/libwbclient/wbc_pam_sssd.c:52:5: error: statement with no effect [-Werror=unused-value]
         WBC_ERR_WINBIND_NOT_AVAILABLE;
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    src/sss_client/libwbclient/wbc_pam_sssd.c:53:1: error: control reaches end of non-void function [-Werror=return-type]
     }
     ^
    
    Related to:
    https://pagure.io/SSSD/sssd/issue/3461
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>