scons: Build nir/glsl_types.cpp once.

scons: Build nir/glsl_types.cpp once.

Undoes early hacks, and ensures nir/glsl_types.cpp is built once, and
only once.

The root problem is that SCons doesn't know about NIR nor any source
file in the NIR_FILES source list.

Tested with libgl-gdi and libgl-xlib scons targets.

Reviewed-by: Brian Paul <brianp@vmware.com>