meson: use project build root for doxygen output

meson.build_root() returns the parent's build directory if Wayland is
a subproject. This fails with:

    error: tag OUTPUT_DIRECTORY: Output directory '<parent-build-dir>/doc/doxygen' does not exist and cannot be created

Instead, use meson.project_build_root(), which returns the subproject's
build directory.

Signed-off-by: Simon Ser <contact@emersion.fr>