nouveau: add idep_nir_headers as dep for libnouveau

Fixes a compilation error when building libnouveau:

In file included from ../src/gallium/drivers/nouveau/nv50/nv50_program.c:25:
../src/compiler/nir/nir.h:1115:10: fatal error: nir_intrinsics.h: No such file or directory
 #include "nir_intrinsics.h"
           ^~~~~~~~~~~~~~~~~~
           compilation terminated.

Fixes: f014ae3c ("nouveau: add support for nir")
Signed-off-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
(cherry picked from commit 8c3ace69)