graph-depends: ignore the 'target-purgelocales' target

Otherwise, graph-depends tries to call 'make target-purgelocales-show-depends',
which does not exist, as 'target-purgelocales' is not an actual package.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>