Adding missing cstdint include in stringutils

[redstrate]

Description

This was failing to build on my machine, since I guess one of the std headers did not include as expected.

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

Test Configuration:

• OS: Arch Linux

Checklist:

• [X] My code has been formatted with the clang format file