d/control: record lz4 build dependency

Since Ubuntu Eoan the kernel compression was changed from GZIP to
LZ4, due to slightly faster load times vs. a 25% size increase
trade-off (e.g. 5.0 had ~ 8, this one has ~ 12 MB; *but* the initrd
stays roughly the same size, and that one is 5 times bigger anyway)

If we want to keep that is in the stars, but for now correctly
document the build-dependency to LZ4.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

debian/control.in

@@ -23,6 +23,7 @@ Build-Depends: asciidoc-base,
                libssl-dev,
                libtool,
                lintian,
+               lz4,
                perl-modules,
                python-minimal,
                rsync,