Fix include_directories when using Atk as a subproject

Luca Bacci requested to merge lb90/atk:fix-meson-using-as-subproject into master

Project that use Atk include the Atk headers as:
#include <atk/atk.h>
and not:
#include <atk.h>

Currently projects that use Atk as a subproject fail to compile because declare_dependency() uses include_directories: atk_inc. Change to include_directories: root_inc.

Edited by Luca Bacci

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