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/libosmocore-dev.install b/debian/libosmocore-dev.install
index ece5b08..7c5a789 100644
--- a/debian/libosmocore-dev.install
+++ b/debian/libosmocore-dev.install
@@ -1,6 +1,6 @@
 usr/include/*
-usr/lib/lib*.a
-usr/lib/lib*.so
-usr/lib/lib*.la
-usr/lib/pkgconfig/*
+usr/lib/*/lib*.a
+usr/lib/*/lib*.so
+usr/lib/*/lib*.la
+usr/lib/*/pkgconfig/*
 usr/share/doc/libosmocore/*