Use pid_t for meta_window_get_pid()

The return value of meta_window_get_pid() changed again and it now
returns pid_t, which usually just resolves to int on most platforms. We
should still use pid_t now though, so do that.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1188

src/shell-app.c

@@ -1158,7 +1158,7 @@ shell_app_get_pids (ShellApp *app)
   for (iter = shell_app_get_windows (app); iter; iter = iter->next)
     {
       MetaWindow *window = iter->data;
-      int pid = meta_window_get_pid (window);
+      pid_t pid = meta_window_get_pid (window);
 
       if (pid < 1)
         continue;