Add example osmo-mgw configuration file for Abis/E1

In this example, we are using the first span (0) of the first DAHDI card
and use it as 'trunk 1' in the MGW.

Change-Id: I0a97da5163a94379b327403b1258696855836bad
diff --git a/contrib/osmo-mgw.spec.in b/contrib/osmo-mgw.spec.in
index fc618da..b08a598 100644
--- a/contrib/osmo-mgw.spec.in
+++ b/contrib/osmo-mgw.spec.in
@@ -113,6 +113,7 @@
 %dir %{_docdir}/%{name}/examples
 %dir %{_docdir}/%{name}/examples/osmo-mgw
 %{_docdir}/%{name}/examples/osmo-mgw/osmo-mgw.cfg
+%{_docdir}/%{name}/examples/osmo-mgw/osmo-mgw-abis_e1.cfg
 %{_bindir}/osmo-mgw
 %{_unitdir}/osmo-mgw.service
 %dir %{_sysconfdir}/osmocom