meson: Fix disabling gtk_doc
Without this, even passing -Dgtk_doc=false will cause documentation to be built if gi-docgen is available.
Fixes: 82119a6b ("gi-docgen: Initial commit")
Without this, even passing -Dgtk_doc=false will cause documentation to be built if gi-docgen is available.
Fixes: 82119a6b ("gi-docgen: Initial commit")