graph-depends: add to exclusion list

Add the root-password internal target to the exclusion list.

Fixes failures like:
    Getting dependencies for [... 'target-root-passwd' ...]
    Error getting dependencies [... 'target-root-passwd' ...]

Which is easily singled out with:
    $ make target-root-passwd-show-depends
    make[1]: *** No rule to make target `target-root-passwd-show-depends'.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>