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

Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7696)

(cherry picked from commit 76bc401cc63219a462224884cb4af787e17725ed)

.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