timeshift: Fix GCC 14 build

TeeJee.Process.c:1134:9: error: implicit declaration of function 'kill'

pkgs/applications/backup/timeshift/unwrapped.nix

···
     xapp
   ];
 
+  env = lib.optionalAttrs stdenv.cc.isGNU {
+    NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration";
+  };
+
   meta = with lib; {
     description = "System restore tool for Linux";
     longDescription = ''