6.3: don't build linux-image-librem5 binary package for linux-next-librem5

debian packaing was reworked for 6.3 quite a bit, see git log --oneline v6.2..v6.3 scripts/package/* and dpkg-buildpackage now builds linux-image-librem5 in any case. We must not build that for linux-next-librem5 source packages though.