meson: correct dependency on nir headers.

Avoid build race if nir_intrinsics.h is not generated yet.

Signed-off-by: Tobias Nygren <tnn@NetBSD.org>
Signed-off-by: Maya Rashish <coypu@sdf.org>
22 jobs for !1908 with meson-nir-nouveau in 22 minutes and 20 seconds (queued for 3 seconds)
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