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/libosmocore6.install b/debian/libosmocore6.install
index d134902..b73331b 100644
--- a/debian/libosmocore6.install
+++ b/debian/libosmocore6.install
@@ -1 +1 @@
-usr/lib/libosmocore*.so.*
+usr/lib/*/libosmocore*.so.*