man: fix out of source build
man: fix out of source build

Use build and source directory variables for correct paths. This
distinction is important for out of source builds.

Signed-off-by: Alexander Stein <dbef83f64948dc68aad31596bbf6991154a9d730@mailbox.org>
Signed-off-by: Bartosz Golaszewski <e5d45fddd9040473413b909b802f629a971ad04c@baylibre.com>

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