appimage: Fix hack for building wayland-protocols

* src/appimage/Makefile.am
(appimage_pkg_wayland_protocols_post_install): Only move/rename the
unpackaged sources.

If wayland-protocols was built before then the package install directory
contains more files/directories than just the unpackaged sources. Moving
just the unpackaged sources instead of everything in the package install
directory fixes the mv (which is actually a rename).

• GnuPG-bug-id: T7509