doxygen: Generate Doxygen for NIR

Now, one can do the following to generate and read the nir Doxygen:
cd $MESA_TOP/doxygen
make
firefox nir/index.html

Update v2:
Correct TAGFILES in nir.doxy

Signed-off-by: Elie TOURNIER <tournier.elie@gmail.com>
Reviewed-by: Rhys Kidd <rhyskidd@gmail.com>

[Emil Velikov] v3: Rebase.
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/doxygen/.gitignore b/doxygen/.gitignore
index ea67dd5..4773413 100644
--- a/doxygen/.gitignore
+++ b/doxygen/.gitignore
@@ -10,6 +10,7 @@
 main
 math
 math_subset
+nir
 radeon_subset
 swrast
 swrast_setup