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: I8466713a8d414ea56ca24f6f7119338ad2b98ce5
diff --git a/debian/control b/debian/control
index 6fb34c0..8e0e079 100644
--- a/debian/control
+++ b/debian/control
@@ -7,7 +7,8 @@
                pkg-config,
                autotools-dev,
                libosmocore-dev,
-               libosmo-netif-dev
+               libosmo-netif-dev,
+               osmo-gsm-manuals-dev
 Standards-Version: 3.9.8
 Vcs-Git: git://git.osmocom.org/osmo-mgw.git
 Vcs-Browser: https://git.osmocom.org/osmo-mgw/
@@ -33,3 +34,12 @@
 Multi-Arch: same
 Depends: libosmo-mgcp-client5 (= ${binary:Version}), ${misc:Depends}
 Description: libosmo-mgcp-client: Osmocom's Media Gateway Control Protocol client utilities
+
+Package: osmo-mgw-doc
+Architecture: all
+Section: doc
+Priority: optional
+Depends: ${misc:Depends}
+Description: ${misc:Package} PDF documentation
+ Various manuals: user manual, VTY reference manual and/or
+ protocol/interface manuals.