Allow public_key:pem_entry_decode/2) to handle AES-128-CBC ciphered keys

Private keys generated by modern versions of ssh-keygen stopped are ciphered with AES-128-CBC instead of DES-EDE3-CBC.

Since DES-EDE3-CBC ciphered keys are handled, and the underlying support for AES-128-CBC is already present, it seems a bug of omission that AES-128-CBC ciphered keys are not.