Add multiarch metadata to lv2

Release: Artful Aardvark 17.10
Package version: 1.14.0~dfsg1-1

As far as I can tell, the package lv2-dev has no architecture-dependent files at all, and can be marked Multi-Arch: foreign, and lv2-examples puts .so files outside the architecture-dependent folders, making it correctly Multi-Arch: no by default (and I don't know if that would be easy or useful to fix and make Multi-Arch: same).

In addition, it's possible the packages python and perl in Build-Depends can be marked as :any, as these seem to be invoked as binaries, which should work properly regardless of the underlying architecture.

I am not currently able to test a cross-build, as the examples depend on libgtk2.0-dev, which currently lacks multi-arch support, and I'm not sure how/if it's possible to test easily without gtk support. multi-arch libgtk2.0-dev was the only remaining dependency I was unable to install.