Revert "add MANIFEST.generated to .gitignore"

This reverts commit 5f89700d9c98f0f8216f72edadfc6134726c324c.
Not generated with parrot 5.0, since https://github.com/parrot/parrot/issues/922
install dynpmc headers, as parrot 5.0 does so

See https://github.com/parrot/parrot/issues/922