Bugfix: Update target include directory for t8_helper_functions

[saustinp]

Describe your changes here:
This PR fixes a bug in the build system where CMakeLists.txt was specifying an incorrect target include directory for src/t8_helper_functions. By correcting this line, the project now correctly finds the t8_helper_functions headers in environments where it previously failed. Fixes #2219.

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[sandro-elsweijer]

Hey Sam,

thank you for your submission!
Our open source license requires, that every contributor places his contributions under said license. Can you therefore please copy one of the license statements under doc/ and adapt it to your name. This only needs to be done in the first PR. After that we can merge :)