Revert "update to use gnutls28"

This reverts commit 763aab538dcbd61aa6c8c7e6c61c1e42316508ca.

Cross build of gnutls28 fails:

make[2]: *** No rule to make target '../gl/libgnu.la', needed by
'libgnutls.la'.  Stop.
temporarily take libgnutls26 from snapshot.debian.org