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.

pkgs/applications/editors/vscode/generic.nix

@@ -1,5 +1,5 @@
 { stdenv, lib, makeDesktopItem
-, unzip, libsecret, libXScrnSaver, wrapGAppsHook
+, unzip, libsecret, libXScrnSaver, libxshmfence, wrapGAppsHook
 , gtk2, atomEnv, at-spi2-atk, autoPatchelfHook
 , systemd, fontconfig, libdbusmenu
 
@@ -63,7 +63,7 @@ in
     buildInputs = (if stdenv.isDarwin
       then [ unzip ]
       else [ gtk2 at-spi2-atk wrapGAppsHook ] ++ atomEnv.packages)
-        ++ [ libsecret libXScrnSaver ];
+        ++ [ libsecret libXScrnSaver libxshmfence ];
 
     runtimeDependencies = lib.optional (stdenv.isLinux) [ (lib.getLib systemd) fontconfig.lib libdbusmenu ];