vscode*: add libxshmfence to buildInputs

[flokli]

Like recent chrom[e,ium], Electron >= 12 applications need libxshmfence.

vscode is preparing to switch to electron 12.0.0 (https://github.com/microsoft/vscode/tree/electron-12.x.y).

This introduces libxshmfence to the buildInputs of generic.nix, allowing
custom builds to reuse the same generic Nix. We'll also need this code
as soon as a release with Electron 12.x is out.

Motivation for this change

vscode with native wayland support

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
• Fits CONTRIBUTING.md.

[r-rmcgibbo]

Result of nixpkgs-review pr 116553 at 510171f run on aarch64-linux 1

3 packages built successfully:

• vscode
• vscode-with-extensions
• vscodium

---

Result of nixpkgs-review pr 116553 at 510171f run on x86_64-linux 1

3 packages built successfully:

• vscode
• vscode-with-extensions
• vscodium

[Synthetica9]

LGTM. Could maybe even be added to AtomEnv if they're also planning to switch to E12?

[flokli]

@Synthetica9 I'm not sure about atoms plans on E12 bumps, but we can do that as a followup.

[turion]

vscode with native wayland support

Does it make sense to expand the integration test (nixos/tests/vscodium.nix) to include a wayland case?

[flokli]

We'd need to have a public binary with this code first.

My current testing case had src is pointing to a binary I built together manually, outside a Nix build, due to a pure vscode build inside nix being a bit out of reach.