configury: add luaexec module uninstall hook.

* Makefile.am (uninstall-luaexec-modules): Uninstall luaexec
modules, even though we didn't install the .la file that
Automake's uninstall rules rely on.
(uninstall_hooks): Add uninstall-luaexec-modules.
(uninstall-hook): Depend on uninstall_hooks.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>