debian: Move to multi-arch support in libosmocore

Enable multi-arch support in libosmocore. This means the package
can not be built on squeeze anymore.

The concept of "foreign" is not really well documented but I
think I use it correctly here.
diff --git a/debian/libosmovty1.install b/debian/libosmovty1.install
index 1b993e8..fbf6a5f 100644
--- a/debian/libosmovty1.install
+++ b/debian/libosmovty1.install
@@ -1 +1 @@
-usr/lib/libosmovty*.so.*
+usr/lib/*/libosmovty*.so.*