libjson-rpc-cpp fails to cross build from source, because the build
requires a native jsonrpcstub to be installed but there is not. To get
there, I am proposing to add a recursive build dependency conditional to
cross compilation at the same time as marking the relevant package
Multi-Arch: foreign. In order for this to work argtable2 needs to be
coinstallable #1101321, but this patch can be applied independently.

Helmut