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/compat b/debian/compat
index 45a4fb7..ec63514 100644
--- a/debian/compat
+++ b/debian/compat
@@ -1 +1 @@
-8
+9