Makefile.local: Ensure to kill child processes at devserver target exit
Makefile.local: Ensure to kill child processes at devserver target exit

This change ensure that child processes created by the make target named
run-django-webpack-devserver will always be killed when exiting.

It fixes the issue of webpack related processes still alive when
the django runserver command exits with errors.

SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui