BUILD: Fix linking cwrap tests with -Wl,--as-needed
    
      CCLD     responder_common-tests
    .libs/libsss_util.so: undefined reference to `sss_base64_encode'
    .libs/libsss_util.so: undefined reference to `s3crypt_gen_salt'
    .libs/libsss_util.so: undefined reference to `sss_base64_decode'
    .libs/libsss_util.so: undefined reference to `s3crypt_sha512'
    collect2: error: ld returned 1 exit status
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>