build: add libnir.la

build: add libnir.la

If we want to use NIR from state trackers that don't already pull in the
whole of glsl (ie. anything other than mesa state tracker), we need a
separate more minimal libnir.  Possibly NIR should be better split out
from glsl, but for now, generate a second smaller libnir.la for those
who just want NIR but not all of glsl.

Signed-off-by: Rob Clark <robclark@freedesktop.org>