add debian packaging related information

Using the debian directory in this commit, it is possible to generate
libosmocore and libosmocore-dev packages (dpkgs).
diff --git a/debian/compat b/debian/compat
new file mode 100644
index 0000000..7f8f011
--- /dev/null
+++ b/debian/compat
@@ -0,0 +1 @@
+7