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/doc/manuals/Makefile.am b/doc/manuals/Makefile.am
index 901cffa..0faebf3 100644
--- a/doc/manuals/Makefile.am
+++ b/doc/manuals/Makefile.am
@@ -12,5 +12,6 @@
   VTY_REFERENCE = osmomgw-vty-reference.xml
   include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.vty-reference.inc
 
+  OSMO_REPOSITORY = osmo-mgw
   include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.common.inc
 endif