Include stdint.h in avr/i2c_master.h

[waffle87]

Description

Without this directly included here, errors are likely to occur when making use of data-driven configuration and omitting config.h/other files.

Types of Changes

• Core

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).

[KarlK90]

Thanks!