This was copied over from GTK's buildscript. GTK uses this because they build multiple gir files and need
gi-docgen to find them all. libadwaita only builds a single gir file, so there's no need to add any additional paths.
Besides, the path used here was broken (it should have been
../../src), and nobody has noticed.
Resolves #680 (closed)
Signed-off-by: Sergey Bugaev firstname.lastname@example.org