Pay with PayPal: Fix currencies lib loading

[mmtr]

Changes proposed in this Pull Request:

In #17381 I moved the new Jetpack_Currencies class from the root of Jetpack to _inc/lib but forgot to load it correctly using jetpack_require_lib when rendering a simple-payment shortcode. This PR ensures the lib is loaded as expected.

Jetpack product discussion

N/A

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Spin a JN site running this branch.
• Set up Jetpack and purchase any paid plan.
• Go to WP Admin > Posts > Add new.
• Insert a Pay with PayPal block and fill it with data.
• Switch to the code editor in order to grab the ID of the internal simple-payment object created.

• Switch back to the visual editor and insert a Shortcode block.
• Enter [simple-payment id=<ID>].

• Publish the post.
• Make sure the post is published successfully.
• Visit the published post.
• Make sure the shortcode is rendered successfully.

Proposed changelog entry for your changes:

N/A.

[jetpackbot]

Scheduled Jetpack release: November 10, 2020.
Scheduled code freeze: November 3, 2020

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-17625

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

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