scons: gallium/auxiliary: Unconditionally compile NIR regardless of LLVM

NIR sources are not depending on LLVM (anymore?) as can be seen in the
equivalent unconditional inclusion of nir/ source files in meson.build.
Symbols in these files are necessary to compile softpipe:

    Linking build/linux-x86_64-debug/gallium/targets/libgl-xlib/libGL.so.1.5 ...
    /usr/bin/ld: build/linux-x86_64-debug/gallium/drivers/softpipe/libsoftpipe.a(sp_state_shader.os): in function `softpipe_create_shader_state':
    src/gallium/drivers/softpipe/sp_state_shader.c:146: undefined reference to `nir_to_tgsi'
    /usr/bin/ld: build/linux-x86_64-debug/gallium/drivers/softpipe/libsoftpipe.a(sp_state_shader.os): in function `softpipe_create_compute_state':
    src/gallium/drivers/softpipe/sp_state_shader.c:435: undefined reference to `nir_to_tgsi'

Fixes: fa483d8c ("android: gallium/auxiliary: Deduplicate nir_to_tgsi.c inclusion")
Closes: #3669

Tested-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Eric Anholt <eric@anholt.net>
Part-of: <!7250>
84 jobs for !7250 with nir-fix-scons in 11 minutes and 9 seconds (queued for 9 seconds)
merge request