[util/vendor_hw.py] Add signoff when using --commit

Tested by re-vendoring ibex with --commit and checking that the signoff
line was created.

Closes #911.

Signed-off-by: Garret Kelly <gdk@google.com>
diff --git a/util/vendor_hw.py b/util/vendor_hw.py
index 038e0e1..f43355b 100755
--- a/util/vendor_hw.py
+++ b/util/vendor_hw.py
@@ -261,7 +261,7 @@
         cmd_add = ['git', '-C', str(repo_base), 'add', str(p)]
         subprocess.run(cmd_add, check=True, encoding='UTF-8')
 
-    cmd_commit = ['git', '-C', str(repo_base), 'commit', '-F', '-']
+    cmd_commit = ['git', '-C', str(repo_base), 'commit', '-s', '-F', '-']
     try:
         subprocess.run(cmd_commit,
                        check=True,