graph-depends: ignore the 'target-generic-dont-remount-rw' target

The graph-depends script tries to call 'make target-generic-dont-remount-rw',
which doesn't exist since 'target-generic-dont-remount-rw' is not a package.

See also the comments for commit 72bd61e5b8c2094378.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>