mesa: don't specify spirv2dxil output if it's not built

If mesa was overridden to specify a list of gallium drivers that didn't
include d3d12, nix would fail the build for not producing the spirv2dxil
output.

pkgs/development/libraries/mesa/default.nix

@@ -159,6 +159,7 @@ in stdenv.mkDerivation {
 
   outputs = [
     "out" "dev" "drivers" "driversdev" "opencl" "teflon" "osmesa"
+  ] ++ lib.optionals (lib.elem "d3d12" galliumDrivers) [
     # the Dozen drivers depend on libspirv2dxil, but link it statically, and
     # libspirv2dxil itself is pretty chonky, so relocate it to its own output in
     # case anything wants to use it at some point