build: fix installing network-online.target for existing link

build: fix installing network-online.target for existing link

When reinstalling NM on the same location, it would fail with

    Making install in data
    make[1]: Entering directory `/home/data/src/NetworkManager/data'
    make[2]: Entering directory `/home/data/src/NetworkManager/data'
    install -d /opt/test/lib/systemd/system/network-online.target.wants
    ln -s /opt/test/lib/systemd/system/NetworkManager-wait-online.service /opt/test/lib/systemd/system/network-online.target.wants
    ln: failed to create symbolic link ‘/opt/test/lib/systemd/system/network-online.target.wants/NetworkManager-wait-online.service’: File exists
    make[2]: *** [install-exec-local] Error 1

https://bugzilla.gnome.org/show_bug.cgi?id=728965

Signed-off-by: Thomas Haller <thaller@redhat.com>