[gbsyncd] Fix shebang in gbsyncd_startup.py; Make script executable (s…

…onic-net#807)

- Update shebang in gbsyncd_startup.py to `#!/usr/bin/env python3`, as the previous `#!/usr/bin/python` doesn't work with our Python 3 installation
- Make gbsyncd_startup.py executable

syncd/scripts/gbsyncd_startup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 
 '''
 Copyright 2019 Broadcom. The term "Broadcom" refers to Broadcom Inc.