[develop] Fix builds broken by #12240

[daskygit]

Description

Add avr/timer_avr.h as an include to the matrix.c files. This is a quick fix and I guess they all could do with reworking properly.

I've added the include above "hhkb_avr.h" as it's required in that header.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[shelaf]

I think this PR should be merged into the master branch.
If not apply the same patch to the master branch, the corresponding keyboard will fail to build currently.

[fauxpark]

develop will be merged into master at the end of the month anyway: https://docs.qmk.fm/#/breaking_changes?id=when-is-the-next-breaking-change

[shelaf]

I understand everything...
I found out that the same fix as #12240 was made in #13675, so I will open another PR that does the same patch.

[shelaf]

cf. #14072

[fauxpark]

My point was that it may just be worth waiting it out, since the merge date is in just over a week.

[shelaf]

Oh sure. I will wait for the merge.