ubuntu arm64 fails to build packages with mroonga disabled

buildbot fails to build 10.2.12 and 10.3 on arm64 ubuntu due to a change that was introduced in 10.2.12. Since mroonga hits some compiler errors that we don't have workarounds for this was disabled as an engine in the arm64 build, however now the ubuntu build fails because the deb packaging code assumes it should be there.

AFAIK it's our favorite feature of debian packaging, where components are listed explicitly. You've disabled compilation just fine, it doesn't compile anymore, but debian still wants to package it, hence the problem.

A mechanism to silently drop a package if the plugin wasn't built backfired at us just recently with the TokuDB loss, so I don't expect that Sergei is eager to add it [back].

For some plugins which build conditionally we have the conditions in different places – debian/autobake-deb.sh, debian/rules, or whatever, but apparently we don't have one (or the right one) for Mroonga yet. If we want to continue not building it, it needs to be added.