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-window-tracker.c

@@ -336,7 +336,7 @@ get_app_from_window_pid (ShellWindowTracker  *tracker,
                          MetaWindow          *window)
 {
   ShellApp *result;
-  int pid;
+  pid_t pid;
 
   if (meta_window_is_remote (window))
     return NULL;