Update HRTIM module structure.

zephyr/modules/owntech_hrtim_driver/zephyr/CMakeLists.txt

 if(CONFIG_OWNTECH_HRTIM_DRIVER)
   # Select directory to add to the include path
-  zephyr_include_directories(./public_include)
+  zephyr_include_directories(./public_api)
   # Define the current folder as a Zephyr library
   zephyr_library()
   # Select source files to be compiled