Fix non-system-malloc build

* src/xterm.c (x_gc_free_ext_data_private): New function.
(x_gc_get_ext_data): Set it as the private data free function.
2 jobs for master in 48 minutes and 21 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build Images
passed #48027
build-image-inotify

00:45:07

 
  Normal
passed #48028
test-src-inotify

00:03:13