Incorrect libxfce4kbd-private-3 include

Hi,

We found that we are not building the shortcuts plugins in NixOS build and wish to enable it, we got:

shortcuts-plugin.c:31:10: fatal error: libxfce4kbd-private-3/libxfce4kbd-private/xfce-shortcuts-editor.h: No such file or directory
   31 | #include <libxfce4kbd-private-3/libxfce4kbd-private/xfce-shortcuts-editor.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

https://gitlab.xfce.org/apps/mousepad/-/blob/mousepad-0.6.2/plugins/shortcuts-plugin/shortcuts-plugin.c?ref_type=tags#L31

This is unfortunately the same issue and the same cause as xfce4-terminal#224 (closed) (fix) and xfce/thunar#972 (closed) (fix) and replacing it with #include <libxfce4kbd-private/xfce-shortcuts-editor.h> fixes the build.

It will be really great if this can be fixed here as well 🥹

(P.s. I was not able to open MRs due to external user restrictions, will it be possible to get that access so I can upstream some of the downstream build fixes from NixOS? 🙇 )