graph-depends: ignore the 'target-post-image' target

Since the introduction of the post-image mechanism, the graph-depends
script is broken: it tries to call 'make
target-post-image-show-depends', which doesn't exist since
'target-post-image' is not a package.

So we should simply ignore this 'target-post-image'.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>