ppp: add eap-tls patch

[teto]

else NetworkManager-l2tp fails with:
checking whether EAP-TLS patch has been applied to pppd... no
configure: error: EAP-TLS patch has not been applied to pppd

This check has been added in nm-l2tp/NetworkManager-l2tp@56e82af,
i.e. l2tp version 1.7.0.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[falsifian]

Thanks!