doc: fix Doxyfile
doc: fix Doxyfile

The header path was not updated in the Doxyfile after recent directory
structure changes.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,clemensg/libgpiod,brgl/libgpiod,clemensg/libgpiod,brgl/libgpiod,brgl/libgpiod