Fix compiler warnings when X11 autolaunch and launchd are both disabled

Fix compiler warnings when X11 autolaunch and launchd are both disabled

From the department of "if it isn't tested, it doesn't work". I tried
compiling dbus without an assortment of optional features:

    in_builddir ~/build/dbus/legacy ${MR_REPO}/configure \
        --enable-developer --enable-maintainer-mode --enable-tests \
        dbus_cv_sync_sub_and_fetch=no \
        --disable-selinux \
        --disable-inotify \
        --disable-dnotify \
        --disable-epoll \
        --disable-kqueue \
        --disable-launchd \
        --disable-systemd \
        --disable-libaudit \
        --without-valgrind \
        --disable-x11-autolaunch \
        && ...

and it resulted in -Wunused warnings.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=64362
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Thiago Macieira <thiago@kde.org>