gpauth: limit platforms to *-linux

Since gpauth depends on webkitgtk, it cannot be trivially built on the
darwin platform. Even though webkitgtk is explicitly marked as broken for
darwin, this does not help with showing what platforms gpauth (and
gpclient) is supported on when doing package searching.

Signed-off-by: Rahul Rameshbabu <sergeantsagara@protonmail.com>

pkgs/by-name/gp/gpauth/package.nix

@@ -49,5 +49,9 @@ rustPlatform.buildRustPackage rec {
       binary-eater
       m1dugh
     ];
+    platforms = [
+      "aarch64-linux"
+      "x86_64-linux"
+    ];
   };
 }