UBUNTU: [Config] update Build-Depends: transfig to fig2dev

UBUNTU: [Config] update Build-Depends: transfig to fig2dev

Some time back transfig became a transitional package to fig2dev.
In cosmic this transitional has now been deprecated.  As we are still
copying kernels forward we need to fix this dependency in bionic.
This transition actually occured back in artful, we just missed it.
Switch our build dependency to the "new" fig2dev package.

BugLink: http://bugs.launchpad.net/bugs/1770770
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>