spice-vdagent: make unused-but-set-variable non-fatal for gcc 16

GCC 16 gains stronger analysis of unused variables than previous
versions, and spice-vdagent contains an unused loop variable. We
submitted a fix upstream at
https://gitlab.freedesktop.org/spice/linux/vd_agent/-/merge_requests/60,
and it was merged, so we fetchpatch it until it makes it into a release.

pkgs/by-name/sp/spice-vdagent/package.nix

@@ -2,6 +2,7 @@
   lib,
   stdenv,
   fetchurl,
+  fetchpatch,
   pkg-config,
   alsa-lib,
   spice-protocol,
@@ -23,9 +24,23 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-Y+D5vVWXxGOKz9bxDXojVPWZvZ31sx5EMnDKzwfhakA=";
   };
 
+  patches = [
+    # gcc 16's unused variable analysis is stronger than previous versions, and
+    # detects an issue. since vdagent is built with -Werror, this causes a
+    # build failure. a merge request was accepted upstream, but until this
+    # makes it into a release, we fetch the patch.
+    # https://gitlab.freedesktop.org/spice/linux/vd_agent/-/merge_requests/60
+    (fetchpatch {
+      name = "gcc16-unused-variable-device-info.patch";
+      url = "https://gitlab.freedesktop.org/spice/linux/vd_agent/-/commit/e3c74bd3e75a3804692da7d526016a823f6273e0.patch";
+      hash = "sha256-Bf1fwvsQuAVzkN6SNXk7YZxRuhhVInAFxnlrRlavgy0=";
+    })
+  ];
+
   postPatch = ''
     substituteInPlace data/spice-vdagent.desktop --replace /usr $out
   '';
+
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [
     alsa-lib
@@ -40,6 +55,7 @@ stdenv.mkDerivation (finalAttrs: {
     dbus
     systemd
   ];
+
   meta = {
     description = "Enhanced SPICE integration for linux QEMU guest";
     longDescription = ''