python/audit2allow/sepolgen-ifgen: show errors on stderr

This allows test_audit2allow.py to display the errors correctly.

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
diff --git a/python/audit2allow/sepolgen-ifgen b/python/audit2allow/sepolgen-ifgen
index aa1ae8d..e3f67d4 100644
--- a/python/audit2allow/sepolgen-ifgen
+++ b/python/audit2allow/sepolgen-ifgen
@@ -135,8 +135,7 @@
     try:
         headers = refparser.parse_headers(options.headers, output=log, debug=options.debug)
     except ValueError as e:
-        print("error parsing headers")
-        print(str(e))
+        sys.stderr.write("error parsing headers: %s\n" % e)
         return 1
 
     if_set = interfaces.InterfaceSet(output=log)