Update dlt_for_developers.md

[marques-bruno]

Fix error in suggested best-practice code snippet

[michael-methner]

Hello @marques-bruno ,
the declaration of a variable in the for loop is allowed since C99.

[marques-bruno]

@michael-methner the issue here is not the declaration of the variable inside the for statement, but the fact that the lifespan of that variable is bound to the scope of the for variable.
If this code might compile depending on the compiler, the index variable's content is undefined outside of the loop's scope.
Thus, the log message at line 235 won't work as expected:

LOG("Loop count: %d", index);

I know this is just a snippet in the documentation, but it is presented as best practice for DLT users, and therefore might be simply copy-pasted into production code. Since some compilers might not detect this as an error, it is probably best to correct it...

[michael-methner]

Hello @marques-bruno , thanks for the hint. that line was hidden in the diff.