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

packaging/rules/debian/rules

@@ -68,6 +68,9 @@ override_dh_auto_configure:
 override_dh_auto_build:
 	# Daemon contribs build + configure
 	mkdir -p daemon/contrib/native
+ifeq (ubuntu_18.04,$(findstring ubuntu_18.04, $(DISTRIBUTION)))
+	sed -i 's/GNUTLS_CONF :=/& --without-zstd/' daemon/contrib/src/gnutls/rules.mak
+endif
 	cd daemon/contrib/native && \
 	../bootstrap \
 	  --host=${HOST_ARCH} \