gitignore: add gbproxy_vty_reference.xml

Change-Id: I33a0df37b4446b12b53c9e1eace8e5766278be1f
diff --git a/.gitignore b/.gitignore
index 64cebba..97c9372 100644
--- a/.gitignore
+++ b/.gitignore
@@ -67,6 +67,7 @@
 doc/manuals/osmomsc-usermanual.xml
 doc/manuals/common
 doc/manuals/build
+doc/manuals/vty/gbproxy_vty_reference.xml
 
 contrib/osmo-gbproxy.spec