gen_links.sh.inc: Fix globbing writing '*.ttcn' to */.gitignore

This was the case under ggsn_tests, which in turn prevents search text
tools like "ag" to find stuff in .ttcn files (because it skips code in
.gitignore).

Change-Id: Iaef3cfd5ae29db352046664ab4949b037ca80307

gen_links.sh.inc

@@ -36,6 +36,8 @@ gen_links() {
 }
 
 ignore_pp_results() {
+	# Avoid using the pattern itself if no file is found in globbing below:
+	shopt -s nullglob
 	for pp in *.ttcnpp; do
 		ttcn_file="$(echo $pp | sed 's/pp$//')"
 		echo "$ttcn_file" >> .gitignore