debian: build GnuTLS --without-zstd on ubuntu_18.04

New versions of GnuTLS need zstd v1.3.5 or later due to their use of
ZSTD_CLEVEL_DEFAULT, but ubuntu_18.04 only has v1.3.3, which is not
new enough.  So we build GnuTLS without it on ubuntu_18.04.

Change-Id: I4128676f2ac750aa77bc56726ef57f8bb3e871e2