gitignore: add ggsn_vty_reference.xml

Change-Id: I39bd36e4bde457b7c7a62ca6aa6d5dadea4051fc
diff --git a/.gitignore b/.gitignore
index 80e4837..507ee62 100644
--- a/.gitignore
+++ b/.gitignore
@@ -79,5 +79,6 @@
 doc/manuals/osmomsc-usermanual.xml
 doc/manuals/common
 doc/manuals/build
+doc/manuals/vty/ggsn_vty_reference.xml
 
 contrib/osmo-ggsn.spec