Don't export the submodules 'boringssl', 'krb5' and 'pyca-cryptography'

Reviewed-by: Matthias St. Pierre <[email protected]>
(Merged from openssl#7696)

Author: levitte

.gitattributes

@@ -7,3 +7,6 @@ fuzz/corpora/**                         export-ignore
 Configurations/*.norelease.conf         export-ignore
 .*                                      export-ignore
 util/mktar.sh                           export-ignore
+boringssl                               export-ignore
+krb5                                    export-ignore
+pyca-cryptography                       export-ignore