manuals: osmo-s1gw: adjust build_publish_manuals
Use the proper command to build osmo-s1gw manuals, so it doesn't fail
with:
+ su build -c autoreconf -fi
autoreconf: error: 'configure.ac' is required
Change-Id: I096fd34fc5819becbc3a08a2dc4065d4913bbc1b
