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/libosmogsm5.install b/debian/libosmogsm5.install
index 12c9180..5e61729 100644
--- a/debian/libosmogsm5.install
+++ b/debian/libosmogsm5.install
@@ -1 +1 @@
-usr/lib/libosmogsm*.so.*
+usr/lib/*/libosmogsm*.so.*