debian: create -doc subpackage with pdf manuals

I have verified, that the resulting debian packages build in my own OBS
namespace (see the -doc packages):
https://download.opensuse.org/repositories/home:/osmith42/Debian_9.0/all/
https://build.opensuse.org/project/show/home:osmith42

Depends: Ib7251cca9116151e473798879375cd5eb48ff3ad (osmo-ci)
Related: OS#3899
Change-Id: I5563e023dc3c8b158a79ce0c9e1478e117b0ec37
diff --git a/debian/osmo-ggsn-doc.install b/debian/osmo-ggsn-doc.install
new file mode 100644
index 0000000..32c2212
--- /dev/null
+++ b/debian/osmo-ggsn-doc.install
@@ -0,0 +1 @@
+usr/share/doc/osmo-ggsn-doc/*.pdf