jenkins-build-common: use 'git submodule update'

Now that we use git submodules in osmo-trx, make sure to initialize and
update them before attempting to build.

Fix for:
  Makefile.am:32: error: required directory ./osmocom-bb/src/host/trxcon does not exist

Change-Id: Ic2b9207b942a8a9edff82737117b2ed9d6d3cfe3

contrib/jenkins-build-common.sh

@@ -102,6 +102,8 @@ have_repo() {
 
   echo "$(git rev-parse HEAD) $repo" >> "$prefix_real/${name}_git_hashes.txt"
 
+  git submodule update --init
+
   cd "$base"
 }