matrix-synapse: add openssl to buildInputs

[lheckemann]

Motivation for this change

It should be included regardless of whether tests are run or not, and
overriding the package to disable tests would break this package up
until this change.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Deployed own matrix server with this change
• Fits CONTRIBUTING.md.

[r-rmcgibbo]

Result of nixpkgs-review pr 123693 at f26200f run on x86_64-linux 1

2 packages built successfully:

• matrix-synapse
• tests.trivial

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Result of nixpkgs-review pr 123693 at f26200f run on aarch64-linux 1

1 package built successfully:

• matrix-synapse