Boost: Fix page cache error on multisite when updating a page

[dilirity]

Fixes HOG-325, related to #45008

Proposed changes:

• Add missing includes in pre-wordpress part of Boost.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

HOG-325

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

no

Testing instructions:

Pre-requisites

• You'll need to test this on a multisite with 2 sites. I'll call these site A, and B. I used Local to setup one (Jurassic Ninja might work as well, but I didn't use it);
• Install Jetpack Beta as a network plugin so you can easily switch the Boost version;

Test that it breaks

• From Jetpack Beta, make sure you're on the trunk or the production version of Boost;
• On site A, activate Jetpack Boost and Page Cache. Make sure page cache works as expected;
• On site B, make sure Jetpack Boost is disabled. Try editing a page and you should see an error.

Test that it breaks

• Keep the same setup as above, but switch to (fix/boost/page-cache/multisite-update-page-fatal-hog-325) from Jetpack Beta;
• On site B, try editing a page and it should work without a problem.

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/boost/app/modules/optimizations/page-cache/pre-wordpress/class-boost-cache.php	29/293 (9.90%)	1.25%	0 💚

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}

[LiamSarsfield]

LGTM! I can confirm this works as expected.