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/libosmocore-dev.install b/debian/libosmocore-dev.install
new file mode 100644
index 0000000..eec0e15
--- /dev/null
+++ b/debian/libosmocore-dev.install
@@ -0,0 +1,5 @@
+usr/include/*
+usr/lib/lib*.a
+usr/lib/lib*.so
+usr/lib/lib*.la
+usr/lib/pkgconfig/*