vscode*: add libxshmfence to buildInputs
vscode*: add libxshmfence to buildInputs

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.

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