fix(gatsby-plugin-sitemap): add missing dependency pify and minimatch

[MartinHelmut]

Description

The npm module pify is required inside the gatsby-plugin-sitemap but not added as a dependency. It is implicitly required by the installation of the dependencies of gatsby-plugin-feed, therefore nothing fails until a strict module handling is used, e.g. by pnpm.

Therefore I added it to the package.json of gatsby-plugin-sitemap to fix this.

[pieh]

Looks good - could you add minimatch as well?

[MartinHelmut]

Ohh minimatch is missing as well? Okay, I will check that and add another commit 🙂

[pieh]

Awesome! Thanks @MartinHelmut!

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