Remove uses of deprecated `Pointer.elementAt`

[dcharkes]

This method has been deprecated for 1.5 years and we tried to remove it. However, it's still being used here.

../../../../../../../.pub-cache/hosted/pub.dev/posix-6.0.1/lib/src/util/conversions.dart:32:32: Error: The method 'elementAt' isn't defined for the class 'Pointer<Pointer<Int8>>'.
 - 'Pointer' is from 'dart:ffi'.
 - 'Int8' is from 'dart:ffi'.
Try correcting the name to the name of an existing method, or defining a method named 'elementAt'.
    final _value = cStringList.elementAt(count).value;

cStringList.elementAt(count).value;
// -->
cStringList[count];

I'll submit a PR.