qapi2texi: Fix to actually fail when 'doc-required' is false

Messed up in commit bc52d03.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1490015515-25851-3-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

scripts/qapi2texi.py

@@ -292,6 +292,7 @@ def main(argv):
     if not qapi.doc_required:
         print >>sys.stderr, ("%s: need pragma 'doc-required' "
                              "to generate documentation" % argv[0])
+        sys.exit(1)
     print texi_schema(schema)