In this tutorial (Using the GP Port in Zynq Devices), Section 'Testing the PL IP with Prepared Software' -> 2 (importing sources from ref_files) the tutorial points us to the example3 directory, which is missing.
I checked in every branch of this repository, and this example does not exists. However, it seems that the desired source file 'hello_pl.c' is present in the example6 directory, you might want to update the tutorial.