src: Use pre-increment iterator syntax
src: Use pre-increment iterator syntax

Cppcheck produces the following warning:

```
string_utils.cc:24:9: performance: Prefer prefix ++/-- operators for non-primitive types.
```

Fixes #4340

alexherbo2/kakoune,mawww/kakoune,mawww/kakoune,casimir/kakoune,casimir/kakoune,alexherbo2/kakoune,casimir/kakoune,mawww/kakoune,jkonecny12/kakoune,mawww/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,casimir/kakoune,alexherbo2/kakoune,jkonecny12/kakoune