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/libosmogb3.install b/debian/libosmogb3.install
index 73ad3d3..4c47425 100644
--- a/debian/libosmogb3.install
+++ b/debian/libosmogb3.install
@@ -1 +1 @@
-usr/lib/libosmogb*.so.*
+usr/lib/*/libosmogb*.so.*