BUILD: Link libwbclient with libdl
    
    dlopen-tests cannot catch it because it has to be linked with libdl
    
    sh$ grep dlopen src/sss_client/libwbclient/
    src/sss_client/libwbclient/wbc_pwd_sssd.c:    ctx->dl_handle = dlopen("libnss_sss.so.2", RTLD_NOW);
    
    sh$ nm --dynamic --undefined-only .libs/libwbclient.so | grep dlopen
                     U dlopen
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    (cherry picked from commit 02bb4f87413c634599ad1d8f384d97605ee53771)
    (cherry picked from commit e5db3ea2958e090162358e706be76382a3d8b941)