nir: resolve nir.h dependency list (fix make distcheck)

nir: resolve nir.h dependency list (fix make distcheck)

Use nir/nir_opcodes.h as is (w/o the absolute path), as it is the target
name used to generate the actual file. Otherwise the target is missing,
the file won't get generated and the build will fail.

Cc: "10.5" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>