dbus: Adjust the location of the jamid binary in the D-Bus service.

This is a follow up to commit 2afa99fd (Install dring to libexec and
edit templates at install time).

* bin/dbus/Makefile.am (do_subst): Replace bindir by libexecdir.

Change-Id: I380e61467ef4f3c4873698c9e2061b9c61370d44