UBUNTU: [Debian] autopkgtest: switch Depends: to build-essential

autopkgtest control Depends: as empty now seems to be an error, we want
to say 'install nothing' so switch it to depend on packages which by
definition are always installed.
Signed-off-by: Andy Whitcroft <apw@canonical.com>

debian/tests/control

 Tests: rebuild
-Depends:
+Depends: build-essential
 Restrictions: build-needed